CSS 2.1 has a more or less useful diff against the previous version - http://www.w3.org/Style/css2-updates/WD-CSS21-20060411-20050613-diff/cover - and against CSS 2 - http://www.w3.org/Style/css2-updates/WD-CSS21-20060411-diff/cover

As far as I've been able to determine, nothing like that exists for DOM 3 events though. :(

See Appendix C for CSS 2.1 Changes.
http://www.w3.org/TR/CSS21/changes.html

A diff-marked version would have been nice, though.

See Dom Level 3 Events Appendix B for changes:
http://www.w3.org/TR/2006/WD-DOM-Level-3-Events-20060413/changes.html

Again, a diff-marked version would be nice.

These? :)
http://www.w3.org/TR/2006/WD-DOM-Level-3-Events-20060413/changes.html
http://www.w3.org/TR/2006/WD-CSS21-20060411/changes.html

Here's a diff file for the CSS 2.1 update: http://nanobox.chipx86.com/xfer/2006-04-11/diff.txt

It looks like the changes were mostly just cleaning up the spec, changing "CSS2" to "CSS 2" and so forth. A few sections are now noted to be non-normative. The non-normative HTML stylesheet appendix has been changed somewhat. Overall, I'm not seeing any big changes. It's probably about ready to progress in its maturity level now.

We've resolved over 150 issues since the last publication, so there's definitely some substantive changes in there (although many of them are clarifications).