I have been searching but haven't really found any info. Has anyone undervolted a i3-2100? Do you need a p67 mobo or doe H67,H61 work?

I can pick up a i3-2100 for about ~30 dollars less than a i3-2100T but I want the power savings.

You're lucky to have low prices for the 2100 in your country.

Not all boards have the same features. Some boards also do not let you use the IGP which would be really bad for power consumption.
I've been able to underboard in software as well in the past but I don't know what would work with your hardware and OS.
Keep in mind that stability is not guaranteed, especially if you don't underclock as well. If you care about that and you have no experience in the matter, let me tell you that stability testing is not trivial.

Consider getting a G620T. You won't have to undervolt or underclock it and you'll get an even lower power consumption than the 2100T at a lower price than the 2100 (the price is much lower here anyway but maybe the situation will be different for you).

Whether or not you can under volt the CPU depends on the mainboard's bios (or UEFI) options.
You have to do a bit of research yourself, there are too many different boards to give a general answer. Xbitlabs is a good source for reviews.
I would advise against a G620T, since it lacks certain features, most notably Quick Sync for video encoding (a major selling point for me).

In addition to HT which is no big deal, it lacks:
Intel® Quick Sync Video
Intel® InTru™ 3D Technology
Intel® Insider™
I have no idea what these are supposed to do. Maybe you could explain in a few words...

In principle, you can also undervolt in software if the BIOS doesn't support it. I'm writing this on an undervolted laptop and the BIOS doesn't support it obviously.

You can underclock it to be almost identical to T version, check Summary of Pentium Processor and More tests. Also some semi related reads, SPCR Intel Core i3-2100T & Core i5-2400S Low Power CPUs

I guess that's what I was wondering, if undervolting and setting the multiplier to a lower number was chipset specific or motherboard specific. It sounds like its motherboard specific which is nice.

I can get a i3-2100 for 99 at microcenter and the cheapest price on the i3-2100T i could find is around 130. Although the G620T does look interesting (83 dollars at newegg). I am probably going to be running a Radeon 6450, so I am not sure that things like 3D, quicksync, etc really matter to me. I have a i5-2500k running at 4.4ghz if I need to transcode something

The only other major difference is that that the i3 series has hyper threading.

http://ark.intel.com/Compare.aspx?ids=53423,53422,53481,

got my i3-2120 undervolted by -0.15v and RAM undervolted at 1.2v running stable at stock speeds (1600Mhz / 3300Mhz) and stable temps with standard cooler of 30degC idle and about 58 under full load (p95 + furmark) or linx 25 cycles, max mem stable. Consumes 20-24w idle (8gig ram, intel 510 SSD, bluetooth and wireless on) and about 55w under full load.

Its a beast of a machine for what I need, mail, web, downloading etc.
Will be going in a HDplex case for silent computing when they are released.

Its on a p8h67-i mini-itx motherboard fYI.

It looks like Asus is the only maker of mini-itx boards that allows undervolting and doesn't have horrible reviews.

These features are all related to video. Quick Sync speed up encoding of video files ten times or so. InTru3D is support for 3D movies and Intel Insider will let you stream movies over the internet in high quality.

I've been thinking about undervolting an i3-2100 myself. Was going to buy a 2100T, but those (just like any non-mainstream component) are not available here. So, is it doable on an H67 board? Is there a recommended board for this?

Also, as I understand it, the CPU lowers its own voltage when it's idling, right? If so, how would the undervolting affect it? Will it still lower its voltage or would it use the value set by the BIOS/UEFI? If the latter, it would diminish the effect of the undervolting.

What about undervolting in software? I believe it's possible under Windows, but what aboutr Linux? i've seen a kernel module that supports this, but the latest Intel CPU it supported is the C2D.

I haven't seen any definite test results but, if you can't undervolt, it looks like you'll get most of the benefits of a 2100T if you can underclock a 2100.

But look if you can find a G620. It's a good bit cheaper and nearly as fast as a 2100T. Power consumption should be only slightly higher (at worst). It lacks some features however (see above). The G620T might also be an option (it comes with a different heatsink apparently).

Getting a G620 here is even harder than getting a 2100T - at least the 2100T appears in the catalog of some stores (even though its not really available).

I think I'll just buy a 2100 and see.