How do I connect a second monitor (DVI) to the T-Bolt based iMac?

If you are only connecting one external display then you can use the Mini DIsplayport to DVI Adapter. The Thunderbolt port can drive Thunderbolt and Mini DisplayPort displays, and using this adapter it can also drive a DVI-D or DVI-I display.

http://store.apple.com/us/product/MB570Z/A?fnode=MTY1NDA5OQ

Depending on the country where you live there are other options available too such as this:

http://www.monoprice.com/products/product.asp?c_id=104&cp_id=10428&cs_id=1042802 &p_id=5106&seq=1&format=2

The video standard in Thunderbolt is mini-DisplayPort. You should be able to use the Apple m-DP to DVI adapter. DVI is reported to be flakey with Thunderbolt Macs. Also the m-DP to DVI is digital DVI, not analog.

If the DVI monitor's resolution is greater than 1920 x 1200 you need a dual link dvi adapter like the following:

Mini DisplayPort | Thunderbolt + USB to Dual-Link DVI Adapter