i'll test the above and report back soon but i'm quite sure its true.
if i receive a call while typing a sms, at the end of the call the sms i was typing disappears hence re-start the typing.
also if i receive a sms while on a call, the audio notification is as loud as your ringer is set. you can't lower the ringer as it will lower the phone call volume too. its bloody irritating at times.

if only they get the small issues fixed, wp7.5 is actually usable but they've taken the iOS template too seriously whereby you start with such basic features missing only to implement them in software updates (hence why mango had a purported 500 updates).