Ok. This has been bothering me for a long time. The AOLSystemMsg that IMs you every time you log on to multiple accounts drives me crazy. I have three different computers I frequently use (1 Mac, 1 Linux, 1 Windows) with either Adium or Pidgin running. Every time I google for a solution I find tons of people complaining and various proposed solutions. All I care about is that it doesn't cause a window or dock icon to flash. Here are my current solutions:

Pidgin

Adium

I used to use an Applescript I hacked up for Adium to ignore AOLSystemMsg (see below) but the internet has provided a better solution! Steve Checkoway came across this page and was inspired to make an Adium plugin to do the ignoring for you. You can get the plugin here. Thanks Steve.

My Old Applescript Hack
This was my old solution... I suggest you use the plugin. see above
I hacked up a simple Applescript to close chat windows by looking at other random Adium Applescripts. I'm sure this can be made to have nicer features but for now I just wanted to close the stupid chat window so i'm not compelled to click on my dock icon to get rid of the red badge telling me AOLSystemMsg IM'd me.