I have an ongoing issue with these messages. In the past, I was getting them only when attempting to auto-upgrade. Then yesterday I was blocked out of the entire admin area. Removing all plugins removed the error and let me get back in, but I still couldn't auto-upgrade. I tried to re-enable plugins one by one, but I started getting "out of memory" errors in the WordPress Development Blog section of my dashboard. These did not appear to be connected to any specific plugins. (That is, I could swap them out, and I still just ended up getting that error at some point.)
I do not see a PHP.ini file. Apparently there isn't a local one, but my host said I could create one, so I tried that. 128M. Didn't help.
I have tried adding the suggested line to my .htaccess file to increase memory, and got a completely different error, which prevented the whole blog from loading at all.
I have tried adding the recommended line to wp-config, to no effect.
I have contacted my web host, and they assure me that, 'though they don't want to increase the memory allocation server-wide (apparently that would cause problems of its own), these fixes should allow for the necessary memory on my own account. (I also have two other WordPress blogs on this same server, neither of which has ever had any problems. This one never had an issue, either, until somewhere around version 2.8.)
Additionally, the numbers that are showing in these error messages don't make sense. It seems to be saying it's running out of memory while there should be plenty left (according to the numbers).
So what gives? (It's looking to me like some kind of WordPress bug.)