Well it's probably not:
Alex - He was logged on a the same time as you were, and SMF boards don't let the same computer/IP log in as two separate users. You might have had two computers running, but that would have been a bit pointless, and they would probably have the same IP anyway.) Has a similar posting pattern though.
Me - I know that.
It could be, then:
Lots of people, but I think these are most likely:
Rudolph - He's hardly ever on, and nether is the admin. If someone is not logged on that often, they may use the wrong name when they do log on. Is a dev.
Annikk - Quite likely, I think. Uses the same style of smiley ( :> , I've never actually seen that elsewhere, niether has Google, but then it hasn't seen it anywhere.) as the admin, who stopped posting around the time of annikk's moving house. Also joined about a month before, so would have been the right time. Fixes and maintains computers, call himself a half Independent Computer Consultant, half Network Administrator and System Administrator. Possible. Might not be quite so likely to make a mistake loggin in, but also might be so used to posting as Annikk, and not logging out first, that he may have just posted without thinking of his persona. Removed the post quickly, so could have been covering his tracks. Posting patterns are not very similar. Haven't seen them on at the same time. Admin is on now, just after Annikk posted, but Annikk isn't...
Someone else - The cop-out. It's just someone else altogether.
Hmmm. I wonder. I'm basing my strongest three suspects on a mistake, a smiley and a cop-out. I doubt that would hold up in a court...
EDIT:added stuff to Rudolph and Annikk.