Well, that came out of nowhere. Although we knew the device existed early this year, today’s arrival of the Dell Aero at AT&T Mobility was a total surprise – at least to us.

And also a bit of a mystery. 

Dell’s Aero smartphone runs not Android 2.2, 2.1 or even 2.0.  The Aero runs Android 1.5 which is so, well… HTC Hero.  Even the Dell Streak tablet runs Android 1.6.  Dell doesn’t think a year-old OS is a problem though, noting that the Aero is running  many “Dell user interface enhancements.”  One would hope.

Features include a 3.5-inch capacitive touchscreen (640×360), UMTS/HSDPA mobile broadband, 802.11b/g Wi-Fi, Bluetooth 2.0, a 624Mhz CPU, a microSDHC card slot (a 2GB card is included, 32GB max), GPS, a 1000mAh Li-Ion battery, and a 5MP camera.

Leaving the disappointing OS version aside, the Aero has a few selling points.  First, it’s only $99 with a two-year agreement, a price which is certain to drop in the coming weeks, at least from third-party retailers.  It’s also a great looking device that’s thin and sleek.  And at only 3.6-ounces, it’s quite light-weight for an Android-powered phone.

You can order the Aero today via Dell.  The handset is slated for direct sale by AT&T “soon.”

This morning T-Mobile announced the upcoming G2 Android smartphone and posted a teaser site for the device.  The T-Mobile G2 will be the first handset from the wireless carrier to work with its growing HSPA+ “4G” data network.

Additional details about the G2 were absent the announcement. We know that the G2 will replace the aging G1, which was the first Android-powered phone on the U.S. market.  We also assume that, like the G1, the T-Mobile G2 will be built by HTC.

T-Mobile announced in July that its HSPA+ network upgrades would be deployed to serve 185 million people by the end of 2010.  Under perfect – but likely never-to-be-seen – conditions, the HSPA+ network can provide up to 21Mbps download speeds to handsets and wireless modems.

We’ll post more information on the G2 as it’s made available.

Just in case you needed reminding, the Motorola Droid X and T-Mobile Samsung Vibrant are here – today.  The two Android-powered smartphones have been anticipated for months; one, a keyboard-free update to a popular Motorola handset, the other the first of many Galaxy S Samsung smartphones coming to United States wireless carriers.

The retail price for the Droid X with a two-year Verizon Wireless contract is $199; WireFly has it for $179.  The Samsung Vibrant is also $199 at T-Mobile after a $50 mail-in rebate; Amazon has it for $149 shipped.

Android fans – today’s a good day.

Already have one of these new phones?  Let us know how you like it in the comments section.