I have a functioning workaround (found at
http://www.macosxhints.com/article.php?story=20030627172202953). I've only tested this with Safari 1.0.
The trick is to use Mac OS X to handle picking the default browser:
1) Open TextEdit. Type a url (any url), then drag off to create an Internet Location File
2) Select "Other" for your www browser in your Notes location prefs
3) Select the Internet location file you created in step 1 as the browser path.
Notes will now use the browser you have selected as default in the Mac OS X System Prefs