The Google Base blog announced Friday that Google Base's editor now has the ability to add rich text. Next time you log into Google Base to add or update your listings manually, you should see a WYSIWYG editor. It does not appear those who do bulk uploads, can pass along rich text.
It's important to note, however, that at this time, we're not enabling any scripting, data input controls, or frames. Also, HTML descriptions are only available for single item posts.