After entering an incorrect password with the SVN product, I was not able to try again with a different password, even after re-installing the extension. Is there some way to clear the cached "bad" password?

OK, I finally figured out that you apparently have to create a DW "Site", which then shows you the SVN context menu (when right-clicking on a file), which contains the "forget password" command.

Yes, indeed, you need to create a DW "Site" to have all features enabled.
Exacctly this is what is show on our product page, in Flash File.

i found the reset password. My problem is similar, but i can't convince it to let me change my username. I've tried blowing away the site and the file, but when i try to checkout a "new" file for my "new" site, it keeps entering the old, incorrect username. Any way to clear the cached, incorrect username?

incorrect username is bad, we cannot reset it, not in this version, I need to ask and propose for next version.