chendur007,
Of course the T-mobile guys won't acknowledge that it is possible to tether your phone to share the $6 T-MobileWeb plan; either they don't know it is possible or they don't want you to do it. After all, it is against their policies.

I briefly used a Nokia 6230 to share the T-MobileWeb with a Sharp Zaurus (an earlier Linux handheld device). While on the Zaurus I had to use configuration scripts and the command line, it did work in the end. On the Nokia tablets it hopefully should be a more straightforward (graphical) setup.
Setting the proxy is the key to making this work. Take a look at the HowardForums wiki for more info. While on the Zaurus I ended up setting the proxy for each individual application, there may be a way to set a "global" proxy when the Tmobile connection is used.

Coincidentally, I also have a 5300 and the T-MobileWeb plan. My first Nokia tablet (n810) is on order, so I definitely have an interest in getting this setup working. While I have no technical knowledge of Linux, my previous research and intermediate networking experience may be helpful.