OEM = technically for an OEM to use. I.e., HP, Dell etc.
It also means no support from MS since you got the disc from HP, Dell, etc. right

?
The reason why this could become a concern is if you activate your OS too many times.
I know with my copy of XP Pro, i believe that after around the fifth time activating (due to many reinstalls/hardware changes), i had to start calling in to manually get it activated.
In short, if you were to run into that issue with your OEM copy, you'd be screwed, since MS will tell you to deal with your OEM.
Others will say the other drawback of OEM is that it's tied to one piece of hardware forever, like the motherboard, e.g., but IMO that's somewhat redundant, since an OEM copy isn't really supposed to be sold to enthusiasts like us in the first place; it's designed for that HP or Dell or custom shop's system they made for you...
This all being said, if i was on a tight budget & wanted to buy Vista, i'd very likely get the OEM version myself.
Just my $0.02; i could be somewhat wrong.