I'll be switching over to Platinum too, once I get it.
As for Diamond, I do feel bad. It does have about 277 or so caught Pokemon on it. I'll probably only transfer the hard to get Pokemon in Platinum, that's easier to obtain in Diamond, if there are any. And of course, all Legendaries will switch over too (so that I can have 2 copies of each legendary). The 12 starters I've collected will also go over to platinum, leaving a copy of each behind. I'll probably use it for breeding starters.
Only reason I'm switching to Platinum is because there's a lot more to do on it: Battle Frontier, new Wifi features, Wifi Nintendo events, new Pokemon forms (and in fact, I think all different pokemon that have different forms can be obtained in Platinum: Deoxys can be changed in Veilstone, Rotom with the secret key, Shaymin, Giratina, etc).