I just found out that PHP 5.3's PostgreSQL PDO driver (as used by Drupal) is broken by the Postgresql 8.0 transition from octal to hex encoding for bytea. It passes the raw hex through to the app, including the leading x , causing PHP's unserialize() function to choke.
I'm using Drupal with Apache and PostgreSQL on Windows (sigh) because I'm writing up a step-by-step hand-holding guide for someone who needs to do some testing against our Drupal database. I wouldn't be using that configuration voluntarily ;-)
Drupal doesn't check that bytea_output is set to 'escape' as a workaround or do a sanity test to detect this fault, so the results are ... interesting:
Notice: unserialize(): Error at offset 0 of 27 bytes in variable_initialize() (line 749 of C:\Program Files\Apache Software Foundation\Apache2.2\htdocs\drupal-7.0\includes\bootstrap.inc).
If anybody else lands up whacking their head against this for a while: just
ALTER DATABASE drupal SET bytea_output = 'escape'
to have your sanity restored.
The amount of breakage being seen in drivers really makes me wonder if the the octal to hex transition should've been done with an opt-in from drivers during negotiation or an explicit SET, rather than just making it globally default. But, hey, hindsight.