I've got a P4 Toshiba laptop, 12" iBook G4, early 2009 MacBook White, iPhone 3G and, 4 days ago, iPhone 3GS.
The wife uses the iBook G4 and iPhone. Most surfing is done on the iPhone, unless she wants a larger screen. She emails & IMs from her iBook, as she uses a hotmail account and iPhone mail app doesn't have an option for hotmail. And she prefers a physical keyboard for IM. To be honest, after saying this, I'm thinking of switching her to a Google Mail account just so she'd use the iBook less, as its quite a waste to be treating the iBook as nothing more than a netbook. I used to use it for way more stuff, running Mactheripper, handbrake, gimp, garageband, and even C&C Generals and other games at one point. I occasionally still use mactheripper on it when she's not using it, just to free up my MacBook to run more handbrake jobs.
Now, all that is done on my MacBook White. I also play Eve Online on it. I've decided to do all my emailing on my iPhone 3GS now, including my office email as well. I think the only time I'll fire up Skype on my MacBook now will be for video calls to relatives overseas.
My Toshiba laptop is now used by my 4-year old daughter exclusively to surf her Playhouse Disney website.
If everything goes according to plan, I'll have an 24" iMac by the end of the year, the MacBook White will be handed down to my wife, and the iBook G4 will become a museum piece. I've got a soft spot for the iBook, having upgraded the harddisk and RAM myself and going through lots of stuff together, and don't wanna see it die under my kids' hands... The Toshiba, on the other hand, is totally expendable.
After I changed jobs, I don't travel as often as I used to, so I don't really need a laptop. It would be great to play Eve, watch movies, and do all the fun stuff on a 24" screen instead of 13". All other portable computing needs can be easily met with my iPhone 3GS.
So that's what I'm doing with all my computers.