If I were you, I would wait. It's almost november, and otherwise you are stuck 2 years to the 3Gs. The 3Gs is much better
but keep in mind the next generation may support LTE and some awesome new features. The 3Gs is relatively small update if you already had a 3G, but the 4th. generation definetly will give the special "wow"-feeling. Apple was able to afford a small update, but in 2010 they have to come up with something bigger (for the 'old school 3G users'). Probably they also have to come up with something bigger in 2011 because 3Gs is pretty similar to the 3G - even though it works much better - and 'rivals' (Sony Ericsson, HTC, Nokia, Palm) will catch up: in fact, they are already trying to do that.
Further if you now upgrade you will have the same question every year, but if you upgrade next June than you can say in 2011 or 2012 "I will upgrade this year again" without to think about "Hm, but it's already november*... maybe I should wait..."
So the conclusion:
do not upgrade because of the following reasons:
1) You will have the same question every year because you are almost in 'the middle' of every iPhone upgrade
2) The 3Gs works much better, but it isn't a revolution: I'm sure the iPhone 2010 will have some awesome features (because 2 small upgrades after each other... don't expect that)
3) Holiday Season: probably you need to pre-order your iPhone
do upgrade because:
1) The extra speed, voice control, compass etc.
- Compass: more and more apps will use it for augmented reality, I really think the compass will play a important role in the feature
- Speed: it will amaze you, the iPhone 3G will be an old grandfather.
2) The iPhone 2011 will have some awesome features: 'rivals' will catch up in both 2010 and 2011 so Apple has to do something special, otherwise they will lose customers. If you buy one now, you can upgrade to the iPhone 2011 without extra cost.
3) AT&T may lose it's exclusive deal, but if not, this definetly will happen in 2011: if you upgrade now, than you can upgrade to the iPhone 5th. gen in 2011 and finally use another carrier... without worry-ing about unlocking or jailbreaking.
The best thing you can do is make a list for your self, the cons and the pros. Look at what fits you best.
* Yeah, I know it's october but against the time you've decided to upgrade it will be november
