I don't think its the ibook as I've had airport expresses since my old G3 ibook. I didn't have any problems when I switched to my G4, but once I upgraded to Tiger I started having skips. From random links on google I'm gathering that this is a known issue, and one of the software updates for airport express/airport cards was supposed to resolve the issue, but doesn't.
My fix was to lay down an ethernet cable to the airport expresses around the house. This pretty much defeats the main purpose of having an AE, but until Apple actually fixes the problem, that's all I really have as a fix.