Problems charging ipod when computer is off (but plugged in)

My Ipod is a third gen and a few days ago, I noticed that my Ipod wouldn't charge when connected to my ibook if the ibook was off (and plugged into the power adapter). At first I thought it was a problem with my ipod, so I took it into the apple store and they shrugged and gave me a new (restored) one. However, when I got it home, I noticed that it had the same problem. So, I ran a series of tests with a number of different ibooks/powerbooks, firewire cables, and ipods to determine where the problem lay:

I tried connecting my ipod to two different powerbooks with two different sets of firewire/docks and it wouldn't charge if the computers were off.

I tried connecting someone else's ipod, and had the same results.

I then tried connecting my ipod to an older ibook (G3) that had experience harddrive failure about a month ago and hasn't had any itunes/ipod updates, and it works just fine, charging when the computer was off like it's supposed to.

I tried connecting it to my old ibook that I haven't used in a few weeks, and again, charged just like it should.

I experienced this problem before I updated to itunes 5, so I don't think it's that (though I wouldn't completely rule it out since it seems to be the source of a lot of problems). Any ideas? Anyone have a similar issue? Again, I'm using gen 3 ipods, two old g3 ibooks (which made it work), a g4 ibook (didn't work), and two g4 powerbooks (also didn't work).