Hello,
I tried to copy this "recipe" into my collections browse file, but it returned a fatal error: get_items() not defined. Is this method outdated, or have I done something wrong (or both)?
http://omeka.org/codex/Recipes/Collection_Item_Thumbnails
Yes, that, like basically all the recipes on the Codex, is for versions of Omeka older than 2.0.
If you're looking to convert it, all the functions used there have straightforward versions in 2.0, and the replacements for each are listed in a big table on the Codex.
Have you checked out the theme deco? It has the code all set up if you wanna just fork it.
Whoa, Deco theme is coooool.