The ports are currently free on the beta, assuming you have bought the original. Seems unlikely that they will change that as "buy once, run anywhere" seems to be a big selling point of Steam.

I guess in terms of incentives to do ports, if Valve plans to launch a Linux based Steam box they can tempt developers with more prominent placing of games with Linux ports, a larger audience (assuming Steambox has good sales) or even flat out make it a requirement to get onto Steam.

There are also current Linux users like myself who are just more tempted to buy a game if it has a Linux port.