SeeSaw
So, I just went to register for a SeeSaw invite, I entered my usual e-mail address, which is iDunno@sommitrealweird.co.uk, and got told it was not a valid e-mail address... erm, yes it is, I use it rather a lot, dammit! So, I changed the localpart from iDunno to one of my less used aliases, and that it accepted... this means that they're checking the localpart and rejecting things with a localpart of idunno, I don't know what other localparts they're rejecting, but that seemed a little harsh...
Ah well, now I'll wait for the invite then I can test :)
Update: Apparently if you put the e-mail address in all lowercase it accepts it fine. sigh.