When the fruits of Google, HTC and T-Mobile’s collaboration were unveiled last months in the persona of the HTC G1 Android Phone, the first smartphone to use Google’s Android mobile OS, it got everyone excited. The device did receive its fair share of critiques, and flaws were pointed out. Nevertheless, everyone hailed it as an exciting newcomer to the land where the iPhone is king. Just how excited, we couldn’t tell up until now.

HTC and T-Mobile felt confident that they could sell about 700,000 G1 units by the end of the year, but lo and behold their initial stock of 500,000 phones was sold-out via pre-order within a few days, and T-Mobile tripled its order, to 1.5 million units. These were promptly pre-ordered as well.

Despite wary reviews, a large number of people are looking forward to the phone. The pre-orders are not counting the customers interested in buying the device, but who have not pre-ordered it. The stock is gone for now, but retail sales should start soon, and with a juicy, two-month window into the Christmas season sales for the G1 could be massive. By comparison, the previous record holder for release-date smartphone sales, the Apple iPhone 3G sold one million units on its opening weekend.

In preparation to the G1’s release, Google has prepared the Android Marketplace (in beta state so far), their own alternative to the Apple App store, and have even held a Developer Challenge with prizes in cash to motivate developers to write for the Android operating system, which will be installed on  number of smartphones to be released in the future.

The question on most people’s minds is whether the G1 will be an iPhone killer, but it’s not that simple. Yes, they’re both smartphones, but while the 3G focuses on media-related features, and the upcoming BlackBerry Storm’s strength lies in a robust business feature set, the G1 plays the card of the Google-developed open, easily expandable platform Android. While the players tend to be complementary, the HTC G1 can be a very appealing contender in the long run, if it can balance its features, and add such missing features as the Bluetooth API.

The G1 will start shipping come October 22, pricing at $179 (a 2 year contract is required).