Installed just by clicking a button in the WriteRoom 2.0 prefs, the command becomes available in any Cocoa app that also uses the NSTextView class to display text. That includes Safari.
That means that any form field in Safari can be edited, in glorious full-screen simplicity, using WriteRoom. Nice for posting to weblogs, but also pretty nice for composing and replying to messages in Gmail. Hit Control + Command + O to open the field in WriteRoom, compose away, and when you return to Safari your text will be there, ready to send.
What you end up with is a hybrid email client that uses your browser just for the browsing bit - for listing and finding messages - and WriteRoom for the editing bit. It’s nice.