There was a zlib bug in PHP 5.2.10 that was fixed in 5.2.11, which zul backported to Ubuntu's 5.2.10 yesterday (php5 5.2.10.dfsg.1-2ubuntu5), but the PEAR symptom relating to zlib (this bug) seems to remain.
If bug #439407 is correct in that the zlib.so extension file fails to install alongside PHP, it's possible that could explain this PEAR issue.
There was a zlib bug in PHP 5.2.10 that was fixed in 5.2.11, which zul backported to Ubuntu's 5.2.10 yesterday (php5 5.2.10. dfsg.1- 2ubuntu5) , but the PEAR symptom relating to zlib (this bug) seems to remain.
If bug #439407 is correct in that the zlib.so extension file fails to install alongside PHP, it's possible that could explain this PEAR issue.