The company was not specific as to which phones would receive updates to Android 2.3, nor did HTC release a timeline on when an upgrade would be expected for release. It is anticipated that newer models will get the upgrade first, and those models include the T-Mobile G2, myTouch 4G, Desire HD, and Desire Z, which is a variant of the G2. The company will most likely roll our new phones with Android 2.3 Gingerbread as well.
In the Android environment, updates are usually controlled by carriers and manufacturers, despite the fact that the OS is free for use by carriers and manufacturers. This problem has led to a problem known as fragmentation where certain devices that do not receive OS upgrades may lack in newer features or functionality. With devices prior to Android 2.2 Froyo, those that did not get the Android 2.2 upgrade were left out of features such as Adobe Flash 10.1 Mobile support and tethering.
Historically, in the case of phone OS upgrades, manufacturers and carriers usually offer one or two major OS upgrades to a phone model. This will encourage users of older handsets to purchase hardware upgrades. Also, as the mobile industry is rapidly evolving, some software features would not be compatible with older hardware models, and this may be an incentive to have customers who desire maximum compatibility and functionality to also invest in the latest hardware.
source:www.gottabemobile.com