One of my clients runs a modest sales site...
(cough! shamlessly promoting bathroomonkey.com! cough!)
...and today was apparently upgraded Wednesday. Both Wordpress (3.1.2) and WP-ecommerce (3.8.2) needed to be updated.
Problem 1: The Wordpress failed with "Fatal error: Allowed memory size of...".
I tried the php.ini trick (it had worked for me in the past - you just make a file named php.ini and put one line: "memory_limit = 64M", and then upload it to the root folder and wp-admin.). No dice this time.
Then I stumbled upon the plugin "Memory Bump". It was written for 3.0 but, long story short, it worked for 3.1.2 as well. Just install the plugin, disable all other plugins, and it worked like a charm. Last step (naturally) is to disable Memory Bump and re-enable your other plugins.
Problem 2: WP-ecommerce upgraded, but woulnt't activate.
This was a nail-biter for me, because at this point the client's site was up, but no products were showing.
Finally I found "The Miracle's" solution here:
Even the developer's support forum didn't have a solution, so I really have to give props to her/him for taking the time to post this. Especially since the blog appears to be mostly about music!