MAgs, the ltd edition should be good for residuals, but I nearly had a brand new "standard S" this Jan, and opted out for a 2nd hand one in the end, cos the potential of a new model may just squeeze them a bit. (look at 996 prices as a guide, nice ones at ?30-311k now due ot new model anouncement!)
The 2003 spec on has a glass window as you probably know, and this is the only worthwhile update on an S for years. The plastic window ones are fine if you look after them. The S is a serious piece of kit, amazing steering and neutral handling as you will have tasted a little already. Believe me, when you really get to push em on, they are very rewarding indeed!
Maybe consider a nice 2000 /2001 model with some toys on. You can get a real nice one for about ?26k say 40k miles on it (say 42k+ new!!), perfect paint etc. Realistically it will be worth 20+k of any eejuts money in 2-3 years time, no matter what happens with the new model.
At the end of the day, no matter how good the new one is, the current model and its predeccessors have been probably the worlds best roadsters by quite a long way!! They will always be worth buying and are easy to sell when the sun comes out!!
I just love em, they are amazingly cheap to maintain through the specialist network, and remarkably robust too. I've had a Porsche for the last 4 years (944sc cab then a boxster S) and I have already spent more in a month tweaking the MK4, than on maintenance for both the porkers in 4 years!!! Go for one!!