Adding a custom field when publishing

Hi!

I’ve just moved No Fat Clips!!! from Blogger to WordPress and I’m spending some time to familiarize with the WordPress API.

I’ve just added a simple feature to add a link in the “metabar” (I’m building on the Barthelme theme, which has that distinctively bar on the left of the posts) to enclose Vimeo clips into a thickbox, rather than opening them in a new window.

I use a custom field to store the URL and the title of the clip (oEmbed converts the URL into the HTML code needed to embed the video).

I’d like to generate this custom field automatically when publishing the post (just like WP does for podcast enclosures) but don’t know where to start, so any tip is welcome.

Thanks in advance,

DeK