Well, if you need it, there certainly is no point in waiting around. I think buying one now and trading it in when the next version is released (or the version immediately after it) is a good policy to stick by. The amount you'll lose will likely not be that much...
But if you are going to wait, you could just go for the top end iMac until Nehalem comes along? There is a good chance that the iMac won't be updated until next summer so you'd be selling the current machine if the new Mac Pro comes along before WWDC.
Of course, if the rumours of a new machine start to turn out to be so strong you could sell a current Mac Pro a week or a few days before the new one is released. Believe it or not, there are folk who don't pay any attention to the rumour mill!!! They have lives...
I managed to do that with my previous iMac and since I get student discount, and I got my RAM for free, I ended up making a tidy little profit on my original purchase!
Oh, and I don't think I would class it as gay to wait around for an update... Just a little extreme, but I waited almost 1 and a half years for the new iMac. Still ended up buying a white one though