I have not received any trackbacks since upgrading to 2.5.1, and I know of several other bloggers who have the same problem (one has a Google PR of 7, and another has a PR of 6, so they are getting links, just not getting credited with them). I found this out from them by doing a poll on Twitter today.
I know for certain that some sites have linked to me since the 2.5.1 upgrade, yet none of the links have shown up as trackbacks. And, they are not showing up at Technorati either.
So, today I upgraded to 2.6 bleeding to see if this might solve the problem. It's too early to tell, but I doubt that it will because it does not look to me like any of the trackback code has changed.