I at first thought we were looking at a changing of the guard type situation, but didn't Desmond get emotional and say something like "I didn't think you would find me here" or something like that. As though he was looking for 'him' but not expecting to meet 'him' on this island?????
If he were expecting a replacement I would think he would expect the replacement to know where to go, and certainly to not blow the hatch off the tunnel.

I'd have to rewatch my tape, but I thought he said, "I thought you weren't coming." or something similar.

Maybe he thought it was a rescue person.

Well, he definitely expecting to meet someone (Are you him?) and the code exchange (what does one snowman say to another snowman), but it seems he is shocked to meet 'him' here on the island. He mentioned to Jack in the stadium that he was about to go on a race around the world. Could it be a 'chase' type of race as opposed to a competition? :) Reminds me of a Monty Python skit, the World Hide and Seek Championship. The contestents would just scatter all over the world and hide.

If he were expecting a replacement I would think he would expect the replacement to know where to go, and certainly to not blow the hatch off the tunnel.

Good point!