So here was my problem: after updating to 2.5.1, I started to get a blank page after updating or posting something new. It would come up to post.php and nothing. Granted, the post itself would be indeed updated or posted (although my Meta Data via Headspace2 wasn't being saved), but beyond that, it was baffling.
I had seen this problem before when I was tapping my resources while still on shared hosting. So I knew this had to be something do to with PHP limitations.
I increased my PHP memory via .htaccess (adding a "php_value memory_limit XXM" line where XX is the amount) and that didn't seem to do the trick. It was while I was looking at my server via FTP that I figured it out: my Sitemaps Generator plugin was taking forever to build the sitemap. I knew this because I could see it happening as I refreshed my server directory. So, poking around in the options, I saw it:
Wunderbar. Click that and my posts were going through. Granted, when I checked out the Sitemaps options I was being told the map wasn't finishing. So I upped the execution time limit to 120 seconds and that so far seems to have done it. I saw a bunch of people in the WordPress forums having this issue for a multitude of reasons and if you found this post then good on you--Googling "blank page wordpress" or the like is ever so much fun.
Why was it working on 2.5 and not on 2.5.1 without these tweaks? Dunno. I have had the image upload whitescreen problem again, even on 2.5.1, but it appears to be intermittent. And I can't quite seem to figure out when hitting Save on a post just saves it and when it sends me (on the same screen/tab) to a preview. I'd love for that to go back to where I can preview when I say preview and save when I say save. Also, it used to be that the checked categories appeared at the top of the list--why did that change? Many, many mysteries.
Update: I think I pinned down my image upload problems...check out my solution here.