i would buy one now. i have a 1st Gen model and sold it for a decent price and picking up a 2nd Gen model...
the thing you can always count on with Apple products is relatively high resale value. September is a good distant away...
I say pick one up and enjoy it. When September roles around and just some features are added - you may feel better that you went ahead and just bought a model that does everything you need it to in the first place.
On the flipside - if the new model has particular features that catch your eye - just sell it your current model. You should have close to zero problems getting it off your hands.
I can't live without my iPod touch...so I refuse to wait till September. If I need to upgrade again...which I doubt I will need to...because practically the current iPod touch is having the features that will be "marketed" on the new ipod touch turned on.
Don't expect a camera and etc...Apple still needs to have particular features seperating the iPhone and iPod Touch. I'm happy using my digital camera taking pictures anyways.
a 64gig model I would say yes to though.