Don't know if this is a ML issue or was present in Lion (which I never upgraded to)
Old setup was 2010ish MBP and only 2006 iMac. Both with SL 10.6.8 and both have two monitors...no issues.
Upgraded to ML on the MBP on release. Realtively painless process. I tried screen sharing today and I get odd responses with the MBP to the iMac. Just so everyone knows, screen 1 is the main iMac screen and screen 2 is the additional monitor. When showing both screens I could see all screen realestate. However, when I changed to only display screen 1 I started having issues. I couldn't get the entire screen to fit within the window nor scroll to show areas not displayed. Sometimes switching between fit in window and no fit, I would get a blank screen. Screen 2 only displayed and all is fine. I noticed on screen 1 that there was a little black bar or open space on the bottom of the window that was exactly the same size that was missing from the top. No ability to fit or scroll would allow the black space to move so that the menu bar on the top of the main monitor would display.
After doing a bit of searching with no luck and scratching my head, I took a look at the Arrangement screen in the display settings. Wouldn't you know it that the main screen (having a smaller resolution than screen 2) was aligned so that the top of each display was even with each other leaving a small space on the bottom of screen 1 compared to screen 2. When I moved screen 1 in the arrangement screen to a more drastic alignment to verify this was the problem...screen sharing crashed when going from both screens to only display screen 1. Tried screen sharing again going from two displays to one, crashed again.
It appears there is some sort of bug or maybe ML and SL screen sharing doesn't like each other when the remote computer has two monitors.
Submitting bug report to Apple but was curious if anyone else has seen this or knows a work around.