They are filtering by user Agent. I heavily use the wifi hotspot feature and tether my n900 to my laptop and noticed this happening around June. My browser of preference is Google Chrome so this message kept on popping up.

If you access secure https webpages on Chrome, you will bypass the "Buy our wifi hotspot tmobile service" page.

Alternatively, I switched to using Firefox which isn't being "user-agent blocked" so I'm free to do any sort of browsing there.

Firefox, Opera, and IE 9 (don't) seem to not be blocked, so use those instead.