There seems to be a lot of confusion out there in the paid search ad space. If the Microsoft & Yahoo do end up closing their deal, which seems likely to me, Yahoo Search Marketing will no longer exist.
The Yahoo Search Marketing console and campaigns you currently run will be either automatically imported into Microsoft adCenter or you will have to manually make that change. Microsoft adCenter will power the ads on Yahoo, Yahoo won't power those ads.
A WebmasterWorld thread seems to have a lot of confusion over this point. Advertisers seem to not want to spend too much time on either adCenter or YSM optimizing their campaigns in fear that the deal will go through and that time would be wasted. I for one think that this is still going to take a really long time, so go ahead and optimize the campaigns. But if the deal does go through and they do begin the migration, the migration will be from Yahoo to Microsoft, not the other way around.
Forum discussion at WebmasterWorld.