I have just been setting up a Ubercart store for a new project on Drupal and was unable to enable the PayPal gateway due to a missing cURL library.
There seems to be quiet a few posts about fixing this with servers running on Windows and not a lot about this for open source OS.
The server in question was Centos running on a VPS.
A guide to resolving this error for both windows and linux is avaialable at the Zend Developer Zone
I recompiled php to finish off enabling cURL with PHP 5.3.1