There are many options for adding depth to the Colts DT rotation. I stress depth because the Colts have made it clear they are not looking for some bigtime starter at DT. They expect Anthony McFarland back next season. McFarland is 30 and a top 10 DT in this league who carries a nice salary cap hit. So, it is depth the Colts are looking for, not another starter per say.
Some options are well known, like signing Tank Johnson or waiting for Warren Sapp to get get. Other options include making a trade, which the Colts are suspect to do since they have no first round pick next year. One potential trade could involve landing Green Bay's Corey Williams, a big, quick DT that the Packers are either going to use as a DE, or just as depth. Williams might cost the Colts a second round pick, which means the Colts wouldn't draft in 2008 until round 3. I'm sure Polian isn't jazzed about that.
The last option is to simply wait until countdown day and grab someone off waivers. Dan Klecko was acquired this way last season. Regardless of the options, I feel pretty confident in whatever decision Polian makes. Guys like Ed Johnson have looked great, and the Colts defense has played brilliantly in preseason without Bob Sanders. Now the The Hitman is back, perhaps only some filler depth is needed.
Though I wouldn't protest with the Colts traded for Williams or signed Tank.