I have a quick question. Did you install postgresql 9.1 on the ubuntu server 9.10 ? If yes, it was a pure installation or update from 8.4 ? If update how did it go ? Where did you get deb package ? Oh, really, I need deb package because install from sources on a lot of server isn't good idea.
Your choices in this situation are limited:
In any case you will need to observe the Postgres upgrade procedure - you can't just dump the new DB binaries on your server and expect it to work with the old
As Craig pointed out Karmic Koala (Ubuntu 9.10) is well past end of life -- you should really be considering an upgrade path to a supported OS (I'm not even sure Postgres will build, or that your system libraries don't have bugs that will cause Postgres to behave badly if it does compile) - If you ask a question about upgrade paths (describing your environment with enough detail) we can probably help you get onto more stable ground.