Over the weekend, Google's shopping search, aka Google Product Search seemed to have started to have issues.
Searching for some popular products brings up no matches found.
For example, a search for [xbox] returns nothing:

Others are complaining that Google Product search returns either zero results or totally irrelevant results.
Google has not yet confirmed a bug nor have they chimed in with a time for a fix.
Forum discussion at Google Web Search Help.
Update: This is now fixed.

Comments:
Chris Faron
08/22/2011 07:43 pm
I'm having problems with products showing active in Google Merchant but showing zero results in a Google shopping search
Johnm
08/22/2011 08:31 pm
Well that's very disappointing because the product search is otherwise very helpful.
Vinny
08/24/2011 01:20 pm
Google's product search algorithm is by far the worse out there. You search for shoes and it outputs digital cameras and rugs. I mean common