I'm not sure if the problem got fixed but until about a year ago I had major issues with md5summer. Whenever I wanted to create checksums for larger files (something llike 300+ MB, such as untracked shows, etc.), it would seem to create them fine, but they would never verifiy. Quite annoying, since I usually don't check my own md5s right after creating them. I never had that problem with mkwact's md5 checksums, so just something to consider (unless this has been fixed, then just ignore my post).
Forgot to say, this was of course reproducable and I wouldn't have noticed if a fellow trader hadn't pointed out to me that one of these files just wouldn't verifiy for him. I then used md5summer again to create a checksum, got the exact same result; with mkw I got a different checksum (one that md5summer also would verify).