Honeycomb will be a significantly more demanding operating system than previous version of Android in the 2.x branch.  Because of this almost all current devices will not be powerful enough to run Honeycomb.  However this will not spell the end of current devices, the technology supporting them, or for that matter further development of 2.x.

Honeycomb devices will be a whole new breed of machine and at a higher cost of course.  Many of the lower end tablets that appeared on the market during 2010 ran on less expensive ARM11 or in some cases ARM9 cpu's but Honeycomb tablets will require chips with Cortex Architecture and significantly greater graphics processing capability plus 3D.  Essentially Honeycomb is not a simple mobile-phone-style operating system akin to what we are all familiar with seeing on smartphones.  It more akin to a desktop operating system in terms of multi-tasking & application features but with the entire look and feel a 3D graphical affair.  

When Google first launched Android one of the things they said about it was that it would run on any size device.  Scalability was a big aspect of the Android project.  The example was given that Android could be used on a 2" screen on a microwave or used as an interface on a 40" plasma screen or for that matter a 100" display wall.  This is still true.  In fact as manufacturers ported Android from device to device in 2010 they realised they had to do very little to change how the operating system worked.  5 inches, 7 inches, 10 inches, made no difference - Android figures it out for you.  And because Android had a few years history under it's belt by the time it came to tablets it meant that the hardware required to power it was easily available and low in price.  So essentially this market will remain further fragmenting this new sector - Android 2.x tablets and Android 3.x tablets.

So if you're wanting Honeycomb then you not only need to wait for honeycomb devices to come out (instead of asking will device xyz be upgradable to honeycomb) but probably want to wait 6 months for the market to settle and prices to come down a little.  If you're wanting a device now at the prices you see now then you're looking at an already established market of devices running Android 2.x which will continue to develop for the lower end of the market as a smartphone operating system.  3.x won't supersede 2.x because 3.x is not for smaller devices or lower specification devices.

There is however an exception at the time of writing (February 2011).  German company Smartbook AG have just launched a 10" tablet running on the NVIDIA Tegra 250, an awesome dual-core Cortex A9 chip that will most definitely run Honeycomb.  Althought this device has come to market in advance of the release of Motorola's Xoom, at present it is running Android 2.2.  This is because Google, although providing Android as open-source, release to prefered developers before making it generally available.  As such the big-brands have access to Honeycomb before the smaller brands.  None-the-less the new Smartbook Surfer 360 is a fantastic tablet right now and Smartbook AG have assured us that it will run Honeycomb and that customers will be able to upgrade in the future.