I’ve noticed on more than once website that, since upgrading to WordPress 3.1, frequent “memory exhausted” errors on Admin pages. It seems like a 32MB allotment just isn’t enough for a WordPress site with a few plugins anymore. Increasing the script memory limit in php.ini (while tweaking the FastCGI process count, in my case, to balance the increase) fixes the fatal errors. 48MB seems to be sufficient, though your mileage may vary.
I wonder what changed that is causing increased memory usage, seeing as the 3.0.x line didn’t have the problem. (Though I recall some of the dashboard widgets occasionally spitting out a similar error on occasion, thanks to SimplePie’s tendency to use a lot of RAM.)