Ok folks.... let's not jump the gun on the next new product. We don't know what this will bring.
Here are the reasons for me to keep my 3GS... My opinion, not anyone elses
Pros
1. Jailbroken (I LOVE IT !!!!) Freedom!!!
2. unlocked
3. Already multitasking
4. Beautiful design
5. Stable phone
6. Extremely satisfied with this model
Cons (Based on leaks)
1. HD will be a battery draining pain (I only need HD on my TV)
2. Front facing camera (I'm not one bit interested)
3. 5 Mega Pixel cam (Wow, that won't make a big difference from 3.0)
4. Removal baterry (It will need to be)
If you plan on HD, coupled with multitasking in excess of 2 programs, then you may need to replace your battery before it's anniversary.
5. The leak pictures displayed a very ugly design.
Having in excess of 2 programs opened on a mobile device is crazy. So the whole multitasking thing is not appealing at all.
Plus I already have that capability at my disposal if I need it.
There will be a speed bump from the 3GS to the new iPhone from the A4 chip. Probably comparable to the one from the 3G to the 3GS.
Also, the iPad has been lauded for its battery. 10 straight hours of video! I know the iPhone battery will be a fraction of the size, but the screen will also be a fraction of the size. The A4 is definitely a power sipping SOC. I only see battery life improving over the 3GS.
The design is your prerogative, but you won't have to look at the new design if you put a case on it.
Other than that, I would expect the new iPhone to be jailbroken/unlocked a month or so after release. That's usually about how long it takes the iPhone Dev Team to work their magic.
Perhaps you could use that month to mull over whether its the right move for you, but I'm just throwing in some info you may have not known.