A receiver with three years' experience may not seem like a ton for the Steelers to let go of, while a draft pick may be what the team needs for the future.
Sanders has shown himself to be a solid receiver, when healthy. While both the Steelers and the Patriots are relatively thin at the receiver position, both have outstanding quarterbacks who can throw even average receivers open. That's the theory, anyway. Or at least what we might expect to hear from the Steelers in the event they do not match the supposed one-year deal offered by the Patriots.
A one-year deal seems odd.
Why would they trade a 3rd round pick for a player who would only be under contract for one year?
Another theory is the Patriots could sign Sanders to that one-year deal, and upon signing him, can roll some of that money into a longer deal. The Steelers may not want to match the up-front money the offer sheet is for, but could conceivably match the amount the Patriots would eventually pay for Sanders in 2013 - and Sanders would get more money overall in the form of an extension. It's not as if the Patriots have the most sterling reputation in the league in these matters. And if the Steelers aren't prepared right now to give Sanders a longer term deal, why would he want to stay in Pittsburgh?
This isn't saying this is what the Patriots are doing, but it does make some sense... in a sneaky, underhanded kind of way.