I just posted to the WordPress forum, but this information might be useful here too.
I had setup lines in my .htaccess file to increase the limits on uploads so that I can upload files using the WordPress interface. After the upgrade, these looked like this:
Uploads no longer worked and I was logging errors saying that POST Content-Length exceeds the limit of 100 bytes. Of course, by "100M" I was hoping to be able to upload up to 100 Megabytes. After some poking around, it seems like PHP no longer likes the "M" but it does work if you write out the full value, like this:
Note: While reading up on this, I realized that post_max_size should actually be slightly bigger than upload_max_size.
Oddly, the WordPress interface reports my max upload file size as 100MB regardless of whether I write "100M" or "104857600" (100 * 1024 * 1024). However, the latter fails and reports an error in the user interface while uploading.