According to Variety.com:

MSNBC will carry the first televised presidential debate of the 2008 race when Democratic hopefuls square off in South Carolina on April 26.

Debate, scheduled nearly six months earlier than the first debate of the 2004 presidential race, stretches the campaign 18 months before Election Day in 2008.

An NBC News personality, likely Brian Williams, will both moderate and anchor the debate, which will air in primetime and be streamed on MSNBC.com. Network will announce the moderator and other details later.

Republicans aren’t waiting much longer to get their first debate under their belts. Fox News Channel will air the GOP’s first televised debate, also in South Carolina, on May 15 at the Kroger Center in the state capital of Columbia.

So to sum up:

Democratic debate - Thursday, April 26, 2007

Republican debate - Tuesday, May 15, 2007

Update

If you’re looking for the full 2008 debate schedule, you can find it here:

Full 2008 Presidential Debate Schedule