The last domino has fallen: AT&T has announced that they will offer Android handsets on their network. The last of the major US carriers to sign on with the open-source Google OS devices, AT&T plans five Android-based handsets built by Motorola and HTC this year. Of course, AT&T is the home of the iPhone.

Along with the Android, AT&T announced at CES that they’ll also be offering a couple Palm OS devices as well as adding support for the Palm, Android, Windows Mobile and Nokia app stores. Perhaps wisely, AT&T didn’t bring up the iPhone, though they continue to enjoy an exclusive sale agreement—which apparently wasn’t reciprocal. (Will it be renewed?)

This move by AT&T may be a precursor to signing on with the agreement to sell plans for Google’s new Nexus One. Like most other smartphones, the Nexus One is pretty cool, and Google selling an unlocked version is pretty cool, too—but the price tag (without carrier subsidies and rebates) will effectively keep all but the most tech-covetous shoppers from the Nexus (with the iPhone running as little as $200 vs. the Nexus’s $530).

AT&T will also be adding more cell sites and connections to improve their network, which has been facing a lot of complaints of dropped calls and texts and sluggish download times. (Not to mention the 3G map comparison Verizon’s latest ad campaign harps on.)

What do you think? Will this help AT&T or Google more in the long run?