A few of our support guys (who have XP Service Pack 3) have noticed that when they Remote Assist to certain computers, instead of Remote Assist sending an assistance request, the RA window opens a 'normal' Remote Desktop session inside it, presenting a login window.
It doesn't happen with all remote machines, but if we find one, it happens for every techie who has SP3, every time. The remote machine does not have to have SP3.
Copying the mstsc.exe and mstscax.dll from an SP2 machine to the SP3 machine fixes the problem...
Ah - got it. It's a known issue, with a hotfix:
http://support.microsoft.com/kb/957070