I'm having this same exact problem. Yesterday I was told to switch to to PHP 5.2 and then activate jetpack, then switch back to PHP 5.3. This didn't work and my site wouldn't load. Today I was told "You will need to upgrade the Jetpack plugin to a new version that supports PHP 5.3. The version that comes on your hosting plan does not support PHP 5.3. Please refer to WordPress Codex for in on upgrading your plugins."
I have the most up to date versions of everything. I am on the windows servers (thought it was to do with operating system as well!), but I am not comfortable switching to linux because I am complete newbie with this stuff and do not want to mess my site up completely, especially since I just got it all set up!