This may just be a Debian Squeeze backports thing, but the spell checker in Libre Office doesn't work out of the box, even thought the spell checker dialogue appears and says it's checked the spelling.
The answer is to install the appropriate dictionary extension and restart. (Solution found on the Debian Forum.)
Update: This advice still applies in Debian Wheezy: I used it just now. The link is actually to the Open Office extension site, but the download is recognised by Libre Office as a Libre Office extension.
Update: see this more recent post for a better solution.