For me, the only way to go is rotate. I know that might not be a feasible solution for some people's cabinets, but it comes about as close as you can to a true arcade experience.
I'm using a 19" Betson Imperial CGA monitor. It's actually a shelf-mounted monitor, but couple of small hacks allowed for perfect rotation. I'll post some pictures of the 'guts' at some point soon.
Although the picture doesn't show it, I have two bezels that I swap out when the monitor rotates. But, if you're going to be playing verticals on a horizontal monitor, I would definitely build another bezel...