Yes, that will normally work.

The SIM (or USIM as the 3G SIMs are called) do not determine the network access technology.
A SIM must work in a 2G phone as well as a 3G phone and a USIM must work in a 2G phone as well as a 3G phone.
Above I wrote "normally" due to the fact that a network operator could for example decide to barr 2G subscribers from a 3G network for marketing differentiation reasons.
However, to my knowledge, this is never done the other way around.

The easiest way to find out for sure is to try your SIM in a friends or colleagues 2G phone to see if it works.

What you need to ensure though, is that the other alternative phone is not SIM-locked.
(SIM-locked means it will only allow using a SIM from one specific network operator - and this may not be your operator)

Cheers
Fred

Hi Phil_UK,

This is a really cool tool. Good Job.
Any plans for updates or bug fixes in the near future?
(I'm referring mainly to the restrictions you have listed on the tool description page)

An interesting feature for me (as others have also already mentioned) would be higher frequency blinking of the LEDs, in particular for messages and missed calls. It would ease the quick check of the phone status.

By the way: the Kaiser (= T-Mobile MDA Vario III) LEDs flash exactly the same way as on a previous HTC model I owned called T-Mobile MDA Pro (except that the MDA pro did not have a GPS module and therefore no orange LED flashing on the right hand LED). It could be opened and closed like a laptop to reveal the keyboard and the touch screen could be swivelled around to face outwards - it had 640x480 resolution and ca. 9cm screen - the display was much better than the Kaiser, but the whole thing was huge and heavy. It was Windows Mobile 5 based.

Thanks
Fred