I found changing channels fixed the skipping issue.
The prob started when I recently updated the OS on my ipad. The tune would drop out for a moment, cut back in, then skip (to catch up?). Rather than resort to downgrading firmware, as suggested elsewhere in this thread, I thought I'd try garryrobson's channel-switching technique first.
I had set my airport express base station channel to 6, so changed it to 12 and, hey presto, not a single skip for the last 4 albums :)
As mentioned elsewhere, manual channel setting is done on the airport base station, which might not necessarily be the airport express unit you're streaming music to.
To change channels:
Open airport utility, select the main airport device, click edit, go to advanced > wifi settings > radio channel. I only changed the 2.4gh one from 6 to 12 and clicked the blue done button, ensuring auto was turned off. The unit then rebooted and everything was fine.
I hope this works for others too - well done garryrobson - sort it out apple!