The DSi's common key is an unknown encryption key (using the AES-CBC-128 algorithm). It is used to decrypt the title key in a ticket.
Only Loopy has substantially claimed to have found the common key, but he has not shared it. He has, however, provided a small amount of information on it. (There are no 00 bytes and the first bit is a 1.)
The common key is *not* in the RAM while either of the currently exploited games are running. The common key is not the same as (and not to be confused with) the Wii's.