I have encountered the same problem described below and spent considerable time this afternoon on the telephone with FTM Support.
This is currently (1532 CDT, 4/19/2013) categorized as a known problem that support is unable to duplicate.
Here is a list of things I have done (at Support's suggestions) that have not cleared the problem:
-verified the newest version of MSIE is installed
-cleared MSIE10 cookies, history et al
-removed all Java, re-installed Java version 7 update 21 (which Java first pushed to me on 4/18/2013)
-verified Norton Internet Security is up to date
-temporarily disabled the firewall and virus protection on my Norton Internet Security.
Just as Steve describes below, I am able to see the details of the potential match, but Merge remains grayed out and no data is pushed from Ancestry to the Search result detail frame on my FTM 2012 page.
As others on this thread have reported, this functionality worked normally for me less than 24 hours ago.
If you discover a fix or work-around please share it with me.