Do you have Arial installed? If it isn’t installed, IE (in Windows at least) will take the next installed font... in alphabetical order. It’s kind of strange though, since Google specifies the general sans serif if Arial isn’t installed, and in your case that should be Helvetica.

Arial is actually kind of nice on-screen, abominable as it may be in print. Just goes to show how different online and print design really is.

It is installed & I already checked Google’s altenative for Arial as well.
Strangely enough, the font that appears on Google is nowhere to be found on my computer (I ran through the entire list trying to identify it, but without succes).
Maybe re-installing OS X might clear things up.

Ill Sans, you probably have some corrupt font caches. Try deleting them with FontFinagler or any such utility.

Linotype Font Explorer does that, too.

Thanks guys, I’ll try that!

I tried Fontfinagler, but everything’s still the same :-(
Any suggestions?