This isn't really a programming question or a server admin question, more a browser behavior question.
In IE (7 & 8 for sure, testing 9 shortly), I'm visiting site A.com, and that opens an iframe which displays site B.com. I'm logged in to site A (have an asp.net_SessionId cookie), and in the iframe I log in to site B (get a cookie from netscaler NSC_AAAC). When I log out of the netscaler session to site B in the iframe (getting a response that sets a bogus value and an expiration date in the past for that cookie), my next request to site A doesn't send asp.net_sessionID cookie, effectively logging me out of site A.
Is this a browser bug? (I think so because FireFox and Chrome do not behave this way) and if so, what can I do to prevent it from happening?