I can now answer my own question!
Searching on Adobe forums I found someone who had a similar sort of problem solved it by clearing the cache in Bridge. I tried this and it did the trick. I have absolutely no idea why - but it worked.
If anyone can explain why this should work I would be interested but at least I have a working tool now.
The Question was:
I want to use Bridge as a front end to Lightroom as described by Jeff Schewe in the LR2 tutorial.
I have LR 2.2, CS4 and am using an EOS 5D MKll. I have downloaded the latest updates. I am using a 24" iMac.
I copied files from my memory card to a folder on the desktop. I can view them in Bridge. I can enter Photoshop and if I use File Open and select a file from the desktop it will open in Camera RAW 5.2.
I cannot however open the same file in Camera RAW from Bridge.
I have put a RAW file from an old Powershot G2 in the same folder on the desktop and that WILL open in Camera RAW from Bridge.
Ouch! I thought this was a CS3 only problem, not seen it in CS4 though it happened half the time I opened a folder in CS3.