Downloaded the installer version of
JetBrains dotPeek 1.0 Full Edition
Build 184.108.40.20644 on 2012-05-09T17:59:52
Ran it with something, not much different from whatever the last version I had was, all good.
Went back to working on code. Did a compare (I have WinMerge as a custom compare/merge tool in VS 2010), and...dotMerge splash, and WinMergeU in the list of files! (no, it's not written in .NET). Went into VS and re-edited the parameters. Still happens. Uninstalled dotPeek. Whew, it went away.
Any idea wtf has happened here?