This has been covered a couple of times over at the GamerCastNetowrk
I think that MicroSoft are actually doing the clever thing. If you want to go all out and get the new model, the HD-DVD drive, the wireless adapter in one go, you can, and it will cost you more. Likewise, if you already own a 360, be it core or premium, then you can also upgrade that. The fact is, nobody is forcing you to have these extras. The other benefit is that it you're on a budget, then you can buy these in chunks. I can afford £245 (although no doubt we here in the UK will get stiffed on that again), I can't afford £425. In a month or two's time I'll be able to afford the HD DVD drive.
I would still rather see one model of each, like Nintendo has done with Wii.