Loki, Loki, as deity of mischief I thought you would've been a bit more lateral, er, vertical, in your thinking. Why can't beholders be flanked by a person directly above them and a person directly below them? As far as the length of eyestalks goes, unless the eye-stalks are able to wrap around to the underside of the beholder, being directly underneath the beholder is safe. It's 6' across. Last time I checked, my shoulders weren't that broad, and most of the PCs are unlikely to be that wide either. I'm speaking as a DM here, too, not some namby player whose PC has been offed by a rampant beholder the DM fudged all-heck out of to kill me. I can kill my players as easy as the nastiest DM; on the other hand, I reward clever thinking. In my book, a PC smart enough and brave enough to position themselves directly (and I mean directly) underneath a beholder gets cut a bit of slack. Next round, the beholder tilts and blasts the PC with 3 eye rays (as per Monster Manual description), but the PC has had at least one round to do something...
Cheers, Al'Kelhar