I finally retired my G5 and switched entirely (for the most part) to my MacBook. It's just too slow to use the G5 for most things these days, especially flash-related ones, and I've found that I've been using the MacBook almost exclusively lately. Personally, I blame flash. It's pretty much impossible to watch Hulu on the G5 and get anything productive done because Flash just eats CPU cycles for dinner. The machine can handle 720 HD, but it chokes on the simplest of flash. But the G5 had a good run; served me well for the past several years.
The G5 is not decommissioned, though; I swapped it for my brother's G4 Mac Mini; it's not a significant speed boost (it's only a 1.8ghz PPC 970fx with a crippled 600mhz FSB and was never top-of-the-line even new), but the system should be more responsive with its significantly larger amount of RAM (maxed out at 4gb). It should carry him a couple more years at least.