In previous version of SQL, it was possible for the server to tie up so many resources, that one more connection was no longer possible. For instance, you could have one process that was blocking all the others, and have no way to open the SQL Server tools to kill it.
That all changed with the DAC. Here's the Microsoft article on how to use it:
Kalen Delaney (Inside SQL Server 2005) has this great query for determining the SPID of the DAC:
FROM sys.tcp_endpoints as t1
JOIN sys.dm_exec_sessions as t2
ON t1.endpoint_id = t2.endpoint_id
WHERE t1.name='Dedicated Admin Connection'; GO