That is one of the things I don't like about boxing and MMA, you should be stripped of the title if you get injured or are expected to be out a long time with illness. I don't buy this whole "interim champion" crap either. It's a fake belt, plain and simple. So the real champion will defend their title against the fake champ.
There is speculation online about what went on between Undertaker and Lesnar. The guy who was interviewing Undertaker said he wasn't in on anything. Lesnar-Undertaker would draw big buys (although Undertaker would have to win, it would be stupid to have his 18-0 streak end to a guy making a one night appearance). I am not sure WWE and UFC could reach a deal though, I don't think Vince McMahon would want to help promote the competition.