mostly depends on the hardware
major versions generally require better hardware but if your device can handle it the manufacturer will probably publish an update.
if manufacturer does not, but device is widely used, cimmunity will port new versions.

i have a samsung galaxy spica, samsung updated it up to 2.1 but i am using a community built 2.3 version.

i also have an advent vega, which i am using to post this message. it's a nvidia tegra2 tablet, advent is yet to publish an android 3.x update but it runs perfectly on a community built 3.2