Same thing happening to me, I'm trying to register my copy of the new splinter cell that I got with the purchase of a video card and it wont let me. I hope this doesn't last long since I am interested in the pre order offers and I wont be able to get them after today I don't think.
I was trying to pre-order MMX as well...
Steam is an option, but you don't get early access to the game that way, as I believe it is exclusive to Uplay,
It seems the preorder is too successful and the logon server(s) could not sustain the load.