If you want the least expensive truly unlimited mobile browsing in the US you should look at Boost Mobile. Boost is Nextel's pre-paid product. Boost is, in my opinion, THE best deal in the country for mobile browsing. I've been a Boost Mobile user for two and a half years, and am quite happy with the service. Unlimited WAP is $6 a month and that's not on top of a monthly charge, this is a prepaid, no contract carrier. You have to put at least $20 into your Boost account every 3 months to keep service active but the $6 for WAP comes out of that. If you aren't making any calls you have unlimited mobile browsing for $6.66/month. In many states, including California, there is no tax on prepaid service. Compare Boost with Virgin prepaid which charges $1/day for WAP that's capped at 500 KB/day. With T-Mobile you have to go to a contract plan at $20/month plus $6/month for unlimited WAP and at least $5/month in taxes and fees. There is "free" browsing on T-Mobile pre-paid but you are limited to just 5 sites CNN, ESPN, ABC News, Pocket Box-office and Amusing Info. With Boost you can go to any of the roughly 500,000 WAP sites in the world.
Boost uses an older technology called iDEN, which was developed by Motorola and never adopted by an major carrier other than Nextel and Telus-Mike in Canada. There are pockets of iDEN use in Latin America, Israel and Korea. iDEN's data throughput (10-20 Kbps) is lower than even GPRS but it also has much lower latency so small amounts of data, like a typical WAP page, actually load at least as fast as under GRPS. I find browsing on my Boost phone is noticeably faster than on a T-Mobile GPRS phone.
All iDEN phones except Blackberries (which not available from Boost) are made by Motorola. Except for the i860 and the soon to be released i855, all Boost phones have old technology WAP1 (wml) browsers. WAP1 has limited support for images and no support for colored text and backgrounds. Fortunately most mobile sites are still available in WAP1 versions and for those that aren't there are transcoding sites that can convert WAP1 to WAP2 on the fly although you lose images and some of the formatting. Although the i860 and i855 are Boost's most expensive phones, they are worth it for the superior browsing experience, in my opinion.
Finally, most Boost phones support JavaME, so they can run game and applications midlets. Boost only officially supports downloading games (and wallpapers and ringtones) from their own portal. The iDEN hacking community is quite active, however and there are ways of "side-loading" content to the phone from your PC using a data cable. If you are interested in loading content or even if you aren't, I suggest you visit the Nextel section at Howard Forums - a great site - if you are at all interested in mobile phones you should check it out.