WAIT WAIT WAIT WAIT WAIT.
If you already own one it is utterly stupid to think about buying the 3g version. About 99 percent of the new features are software, and everyone gets those.
With 3g you really only get faster data, A-Gps (assisted), louder speaker, and maybe better reception.
I like to think the reception is based more on the network than the phone despite how it is marketed, that and i never have issues with the reception on mine. The speakers do suck but it was really meant for headphones. Also, bluetooth headsets totally rock for phone convos so that's also a mute point.
I say wait a few months and see what happens. Remember that hindsight is 20/20 so lets review.
First iphone launch was in June i think of 2007. 3 months later they drop the price 200.00 (probably never going to happen again since 199 seems to be the bottom line now)
Next they release the ipod touch.
Next, sometime before december they phase out the 4gb version in lieu of a cheaper 8gig model and introduce an upgraded 16 gb model at 499.00. The ipod touch drops their price and introduces a 32 gb model.
My predictions are that sometime this year we will see the 8gb get phased out. There will be a 16gb 199 model and 32gb 299 model.
The new ones in 2009 will probably feature a better battery (lots of tech in this field has advanced recently) and more storage. They may also improve the camera or add another though that seems unlikely.
I predict they will almost indefinitely find a way to further improve their own stock software as they seem to have done every update.
The form factor seems pretty set so no needs to worry there (look at ipods to see they dont deviate much).
The 2009 IPhone (to avoid confusion with the 3g moniker) will probably be another slight evolution of the 2008 3g model. There will be more incremental changes. The reason i say wait is mostly because the difference between the 2007 and 2009 iphones will be vastly greater than between the 2007 and 2008 or the 2008 and 2009. Waiting will also save you some money so you can be better prepared to purchase the 2009 version. It will seem like more of an upgrade than this one.
Keep in mind this time around it really feels like apple is pursuing new customers rather than enticing existing customers to upgrade. Their whole must sell x million iphones by this date leads me to believe that once they hit that mark they will focus more on getting these people to upgrade to the 2009 version rather than going after more new customers.
Apple and ATT still have a 5 year contract for exclusivity as far as I know so that makes it open around 2012 to other carriers. Competition in the service field should drive some better pricing schemes among all of the carriers. By then, apple may have revolutionized the iphone or at least i hope the carriers will have revolutionized data transfers to allow for more freedom of features.
Right now, as it stands, its 15.00 more for the 3g data plan, almost double. The idea here is it was 20 for unlimited and 200texts. Now its 30 for unlimited and 5 extra for the 200 texts. 30+5=35 35-20=15 dollars extra for 3g.
Anyway, that is why I will not be upgrading mostly. It still wont hurt to wait it out, see what they do, and see how much different the 3g one is.