Is the bookmark in the left 'collections pane' or the larger right side pane in the bookmarks window? left pane items are alias's.
Had u selected the bookmark or just the text? Have you tried to control+click or right click to select 'delete' from the contextual menu? or use the 'actions' button in the status bar to select 'delete' from the menu?
Is this occurring in a Sneaky Peak? SP's are nightly build can be found here:
http://www.omnigroup.com/application...ad/sneakypeek/