I've been bitten by several variants of this now and can see from the web that many others have too. This thread on Adobe Forums is a good source of information. There are a couple of bugs in Lightroom 2.0 so these steps may help.

Check that you have the latest Photoshop CS3 update installed. Sometimes it doesn't get applied correctly so use Help > System Info from within Photoshop and check that the following are true:

• Adobe Photoshop Version >= 10.0.1
• Camera Raw >= 4.5
• ScriptingSupport >= 10.0.1

Check that you don't have any apostrophes in your folder path anywhere. In my case changing from E:\Photos\James' Camera to E:\Photos\James Camera fixed the problem.

This last one only really applies when you can get the image into Photoshop but not back to Lightroom.

Check that the images isn't appearing in a different volume under the folder browser. Currently there is a bug that differentiates the casing of drive letters so you may find you have two volumes that differ only by case, e.g. E:\ and e:\. This one is a real pain and I hope they fix it soon.

If these don't sort your issue then I suggest you get on the thread linked above and let Adobe know...