HTC launched a new product page on their website that lists the Windows Mobile smartphones they manufacturer that are available unlocked under the HTC brand.
Any unlocked phone is not tied to any particular carrier and can be used with any GSM carrier (though not CDMA carriers like Verizon and Sprint) in the U.S. The two biggest GSM carriers in the U.S. are AT&T Wireless and T-Mobile.
Note that this flexibility comes at a price though (literally). Since these phones are not subsidized by a carrier, be prepared for sticker shock if you’ve never purchased an unlocked smartphone. For example, HTC’s “buy now” link for the unlocked HTC Touch Dual takes you to a Best Buy page that lists a price of $499.99. You might expect to pay between $150 and $300 for similar devices when subsidized by a carrier as part of a two-year contract.