Can't sync more than 5 accounts to ipod

Since about a year ago I've had about 6 iTunes accounts worth of music on my computers (mine, my wife's, my US account and 3 created to get free tunes). These have always synced with my iPods and definitely my iPod Touch, last night I tried to sync my iPod touch after buy a new track with my existing accouont, I got a message saying not all tracks had been sync as only tracks from 5 itunes accounts can be sync to this iPod.

Is this new? If so, is it the latest iTunes or iPod update?

EDIT: OK I found people moaning about this in 2007 so this is obviously not new. What I don't understand is why it's only just started to affect me when I haven't purchased from a different account for over a year!