I'm wondering if anybody's modded / hacked the cellphone from Sims 2 to give it more functionality, a la Sims 3--in other words, I would like my cellphone to act a bit like a computer (blog on it, play games on it, chat on it, etc). I've looked around but can't find anything.

Probably doesn't exist, then. There are smarter computers, but for cellphones I'm guessing you'd need new (or edited) animations. Back in 2003/4 phones weren't as advanced (the closest you came to blogging was sending a MMS with a crappy picture), so they put the more advanced stuff on computers. If you want your sims to play games on a phone-like device, there's always the gameboy device, and for music there's the MP3. You can probably find a default replacement that makes the gameboy look like a phone.

There is a mod to replace the default phone with an iPhone that also allows custom recolors for cases, but it adds no new functionality. Darnit.