Well, I've just downloaded and tried the beta version (4.0.0.55) and it appears so have resolved the issue. (As well as the other I was having of being able to grab from Win7 machines.)
http://www.lansweeper.com/forum/yaf_postst3222_29-March-Remote-screenshot-40-beta.aspx
UPDATE: Some, but not all machines return a blank image (white) when the target screen is locked. Is this expected behavior?