If you've recently upgraded to CiviCRM 4.1, you'll need to upgrade your webform_civicrm as well. Versions 2.3 and below are not fully compatible with Civi 4.1. Version 2.4 is, and will be released in the next couple of days, especially if I get a few comments on this post from people who have sucessfully tested it! The latest -dev is stable and working, so please feel free to download it and try it out on your 4.1 site!
Note: webform_civicrm 2.4 is not backwards-compatible with older versions of CiviCRM, and should only be used with 4.1.x