I agree with Steve but wanted to add a bit.
You don't see your true PR. Google updates PR internally on a frequent (?daily) basis, but we do not see it. They update the toolbar that we see 3 or 4 times each year.
The PR number you see is truncated. Your prior PR may have been 40 and showed as 4. Your PR may now be 39 and show as 3. So a 1 point drop may be extremely minor.
Some details from Matt Cutts: http://www.youtube.com/watch?v=h3Jup5R1MGY
@Panu, I believe you might be missing the relevancy of the Panda affected sites' links and their impact on PR. Most of the sites negatively affected by Panda have links to other sites. The value of those links are now devalued, so the PR of the pages receiving those links would also be impacted.