If it's any clue, here is what FF3.5 reports:
The page isn't redirecting properly
Firefox has detected that the server is redirecting the request for this address in a way that will never complete.
* This problem can sometimes be caused by disabling or refusing to accept
cookies.
Doesn't WP put some stuff in your
.htaccess file for various purposes? This is just an ordinary subdirectory under your root directory? WP might be hitting the root
.htaccess file instead of putting changes in
testblog/.htaccess, or there could be redirects at both the root and subdirectory levels -- check carefully.
....OK, you just posted the root
.htaccess....
I'm a bit suspect of using "redirect 301" rather than RewriteCond and RewriteRule lines, especially when subdomains are involved. Were these redirects added through cPanel or direct editing of
.htaccess. Do you have
testblog/.htaccess (possibly created by WP)? Any
.htaccess files in the subdomain roots?