I just updated to SL a few days ago and also noticed the look-up in dictionary issue.

However, I am not referring to the keyboard shortcut. I'm referring to the contextual menu entry when a word is selected in the rendered zone.

It appears to work in the URL field and in this Quick Reply form box but it does not function on any text in the rendered web page.

The only options I get in the rendered area are Copy, Start Speaking, the Page sub-menu and Inspect Element.

Maybe it's just me?