I'm a bit confused - it seems like DNN is adding massive amoutns of whitespace to the page source - but it doesn't happen on my development server, running a copy of the same files that are on my live server, and pointing at the live server db.

Look at the page source of the forum here - you'll see that there is massive areas of whitespace.

Any idea?

Just filling in a resolved - I was runing a previous version, and the whitespace filter didn't seem to be working.

Updated to latest version, and it's now working fine.  In addition, found the root cause while I was there - Text/HTML module adds whitespace to the source as it gets traffic, eventually making the source files out of control.

The whitespace filter resolves the symptom of this, but the module still has the issue.