Secretly, I'm in love with Desmond myself (oops). But anyway, I was wondering that maybe the seeming thoughtful looks on Des's face are because the lady from the pawn shop told him "that the most important thing that he will ever do is push that button", But wouldn't it occur to Des that what he is doing now in saving Charlie's life on occasion be pretty important too? And that maybe he's thinking that he can change what is considered his"path"?

With reply to Crystal about he said to Jack "You will heal Sarah!"

Perhaps that was him living his life over again as Mother Time had told him he had to. Maybe the whole time we were seeing Desmond before the implosion in the Season 2 finale he had already done all of this before. Maybe this was the reliving that he had to do. Think about it, he got sent into the past by the initial implosion, relives the time up until the implosion again, and then appears naked on the island instead of being sent back in time again, as he had already done it. Now he can see into the future because this would technically be his past and he is having ghost images if you will of what happened when he got sent back into the past. Thats why he is changing things so that these mistakes don't happen again. Only he is not letting time unravel as it should, and is therefore changing destiny. It wouldn't surprise me if Mother time knew he would gain these psychic abilities. That would explain why he couldn't alter his path in the past at all, cos then the future would be different for the Losties, then he wouldn't get the visions and wouldn't be able to change them.