I have an HTC Desire (which I enjoy a lot). However, HTC hasnt updated the device to ICS (they say they will update this year, no specific date).
Can I get a plain android version from Google and install it? What is the difference between the Google Android and the HTC (or other brand) Android?
Technically, yes, you can build the "plain version" of Android: http://source.android.com/source/initializing.html
However, it is difficult to do this for all but a handful of devices (official "Google" developer devices, such as the Nexus line of phones) because every Android phone requires its own drivers and these are not freely available.
This is where projects like CyanogenMod step in: they take the vanilla build of Android, add a few minor features to it, and then try to release it for as many devices as possible. Typically way before any manufacturer gets around to updating the phones (if ever). CyanogenMod 9 is the ICS version of Android, and is tba.
In short, the difference between HTC and Google is that HTC takes the vanilla Android version and then "skins" it with HTC Sense. This is one of the many reasons there is a bit of a delay between when Google release a new version of Android (for example, ICS) and when the manufacturers get around to doing it, because they have to fix their changes/skin to the new version of Android.
First of all HTC will not be releasing ICS for HTC Desire.
You can get plain vanilla Android experience, i.e. Android built for HTC desire using ICS source from here. This ROM is built directly using the code from Google. But Google isn't building it. Awesome guy by the name Sandvold is building it. Please note that this is only a beta version and USB tethering, video and panorama aren't working yet.
I've been using this ROM from past few months and its awesome. Its fast and smooth.
In case you want to follow the development, the development thread is here.
As noted by @Brian, you need to root your phone before you can install this.
No you can't.. Google creates the "plain android version" or vanilla, specifically to work with only a couple of phones (Nexus S and Galaxy Nexus), for the other phones the manufacturer has to customize the Operating System to work with their hardware, and the Carrier (AT&T etc) usually has to pay the manufacturer for that OS version and at the same time asking to have their bloatware put in as well.
Your best bet is to buy the most popular phone or the Nexus line of phones.
For your current phone you should check out http://forum.xda-developers.com/forumdisplay.php?f=628 and see if someone has ported over ICS in a custom rom, you'll need to root your phone, but it's worth it :)
