Using my LongURL PHP 5 library (and optionally integrating with the Libraries API module), a LongURL module would expand short URLs found in comments and nodes and other places. The module itself would be a filter that looks for short URL services then converts any such URLs it finds to expanded URLs. It might have to store a) the services it supports (there is a LongURL.org API call just for this) and b) cache the expanded equivalents in the Drupal database so that the site doesn't make an API call on every page load. It may not make an API on every page load, since filtered text gets cached, but it would definitely call after each time the Drupal cache is flushed.