Apache Rewrite help!

I am trying to write RewriteRule on Apache_1.3.26 to get users web page from another server.

for example if users tries to get web page on

it will get the web page from www.testdomain.com/~username without redirect and users will not be aware of any redirect action or any url changing in the address bar, I want the url to stay the same.

Basically I want, somedomain.com

to become

but get web page from http://www.testdomain.com/~username

I tried this the following

RewriteEngine on
RewriteRule ^/~(.*)$ http://www.testdomain.com/~$1 [P,L]

it work but it works like a redirect and users are aware that the page is being redirected and the url did not stay the same.

I enabled rewrite log and the following entry is log when ~username web page is request.
"go-ahead with proxy request proxy:http://testdomain.com/~thomas [OK]

users are aware that url is redirected. I want the Rewrite rule to work like a proxy, no redirect action just get web page from remote server without url changing.

Can you please, please

Thanks in Advance

Try Ralf Engelschall's rewrite page for a solution (possibly Canonical URLs will help).