This is some great advice. Thanks!
I do have an iPhone4 with ATT. What would I need to do to get it to work in Europe? I'm quite a techy so I don't mind jailbreaking it, but what exactly do I need to get it to make calls? Would the internet service still work? Or is that a suicide for $$$charges.
1.) Make sure you're in Plane Mode, and change your settings so Data Roaming and use is off. Not doing this can KILL YOU in fees (hundreds of dollars!!!).

This HAS happened to my work colleagues!
I don't know how your plan is set up but make sure your phone ain't locked. ATT can unlock the phone for you (for foreign SIM card use) if you're not an absolute new user to ATT. A foreign SIM card will serve to make local calls if you're in a pinch and not at the hotel where concierge can handle things for you (i.e. restaurant appts).
2.) My preferred method of calling back home is using the WiFi on my Smartphone with a great
cheap,
reliable calling card.
As an example, last Fall, I've had over an hour call at 1cent/min talking outside a Paris McDonalds back to the States using the McDonalds WiFi at 4AM after a night's worth of partying. It's insane.
Use a good calling card that mates to a VOIP software. Two calling cards that work well (in order of preference):
a.)
http://www.yupeephone.com/voip_features/how_to_use.htm
b.)
http://www.localphone.com/how_to_call
The services above have recommended FREE VOIP softwares to add to your iPhone on their websites and Yuppee has GREAT customer support to hand hold you in setting-up your phone setting. Test it out at home before using it overseas.
BTW, I use Yuppee as my long distance at home b/c call quality is so good and cheap. The fact that I can do VOIP overseas is just an added great benefit!
3.) For hotels, I recommend using Priceline.com and lowballing bids on their system.
Successful lowball bids on Priceline wipe the floor clean against Orbitz, etc. You can literally save
thousands of dollars on a trip. Learn about bidding strategies on the forums below.
Use the following successful lowball bid databases:
a.)
http://biddingfortravel.yuku.com/ (the best by far IMO)
b.)
http://betterbidding.com/
4.) Bring adapters for using your laptop/electronic devices (Frys, Radio Shack, etc). Verify that your devices power supply works overseas (iPhone is no problem) or you could burn out your device.
5.) Make sure when using your backpack to bring it round to your front in the subway/train (main zippers to one side, not up top). Keep it wrapped to your limbs if you fall asleep on the train (even a mini lock on the zipper carrying important shit). Pickpocketers are ruthless in Europe. Stay frosty. No joke! I don't want to make you paranoid, but just be smart about things and you'll be fine.
Be wary if any foreigner approaches you. It could very well be a scam in progress. Politely avoid and move about on your way. Don't engage in conversation.
6.)
Photocopy your passport page and Driver's License and have them handy on the internet (on email account in PDF/encrypted). This will help if for any reason you lose your travel docs you can access/print them in an internet cafe for reference use at the US Embassy.
7.) Buy train tix a couple of days in advance as they sell out quickly. Trains are the main mode of travel for many and if you wait to the last minute, you could be SOL.
8.) Wear shorts only at the beach. Europeans view shorts as only for use on the beach, on children or on "zeee American". I would wear them on a really hot day, of course, but if you want to blend in, avoid whenever possible.
9.) Don't carry too much Euros at any given time. You can always go back to the ATM. ATMs in Europe almost always
don't charge a fee, which is great! Rely on your credit card (i.e. Schwab Visa) whenever possible.
10.) Before you leave, make sure you
contact your bank and credit card companies when/where you'll be travelling. In that manner, you won't be caught with your pants down when trying to access ATM cash and your bank has put a fraud hold on your account! This DOES happen and has happened to most (including me) who don't plan ahead in this regard.
11.) Try to make a printout of free Wifi places (McDonalds, Stabucks, KFC, etc) close to your hotels/environs. It'll save you the hassle of having to constantly ask. Free Wifi is an amazing luxury to have (free calls, on-the-go travel research, email, etc). Use it and abuse it.

Keep in mind also that at the airport, American fast food places also carry free WiFi.
12.) For tips (i.e. pubs/clubs, events, hostels, etc) on specific cities you visit, you'll find great local flavor info. advice in the forums at:
http://www.tripadvisor.com/
http://www.virtualtourist.com/
13.) Look into travel insurance. It's cheap and takes care of murphy's law stuff that's not covered by your credit card. Two to start off with:
http://www.travelguard.com/#WhyBuy
http://www.travelinsured.com/
And lastly, I do
confirm what others have said in this thread that in Paris you will encounter some of the ugliest behavior witnessed. Parisians treat each other like shit (which is alarmingly WTF hilarious), totally over-the-top....and with English speakers,
some of these people will go out of their way to be THE douche of all douches. Not everyone in Paris is like this, of course, but you should count yourself lucky if you don't run into a
super-douche--50 year olds acting like mean 5 year olds. Regardless, their city is
amazing and definitely worth putting up with stupid adults acting like
jealous children. Unfortunately for me, every time I've gone to Paris, I've witnessed some flavor of uber-stupidity.