Just in case anyone else is seeing this problem, too:
So, like, I had this problem every so often running AM on Mac (OS X 10.7): My cursor seemed to be reacting to imaginary lines in the UI, suddenly flipping between showing as an arrow and as a vertical line with "horns" at top and bottom, typical of a text processing program. Very weird, no rhyme or reason, baffling. This black line cursor is v. difficult to see against AM's dark grey background. And it made it nearly impossible to turn knobs on contraptions. And I also found it very difficult, even when the cursor was showing as an arrow, to grab the right edges of those grey areas in, say, Bassline that indicate the active area of the time line. (The edge that usually triggers the cursor to show as a two-headed arrow pointing left and right, I mean. That dual arrow just wouldn't appear.) And there were some other cursor oddities, including its showing up double when hovered over certain plug-ins.
At first, I thought AM might be causing this. Or that my wireless mouse was losing its "magic." But no. After looking carefully, I saw that the strange behavior was happening in other programs, too, though less so, and even in the Finder. And after some searching, I discovered that the problem is caused, strangely enough, by a couple of browser plug-ins from Adobe that Safari doesn't like. They are AdobePDFViewerNPAPI.plugin andAdobePDFViewer.plugin. Toss these in the trash, restart Safari, and cursor problems are all gone, in AM and everywhere else. As explained here on Apple's discussion board.