Seeing as Nokia have dropped so many b*ll*x in the past, I'd say it stands for Pubic Release.

But all the links I've seen all lead to Youmobile.com or a cut'n'paste on another blog.

Let's face it, the N900 is a developer platform so I think all Nokia are going to do is release updates to QT so developers can continue to develop for the upcoming MeeGo, and probably throw in the odd fix for a bug that we never knew about. i.e "Fixed a bug that prevented sending "Loop-de-loop" commands to Trident missiles over Bluetooth".