Shealy was a fairly highly touted 1B prospect with Colorado who never really materialized. He was traded for Jeremy Affeldt to Kansas City in 2006. He hit .221 with 3 homers in 189 plate appearances in his age 27 season in KC, and despite 79 strong PAs in 2008, including 7 home runs, Billy Butler's emergence made him negligible for the Royals.
He caught on with a minor-league deal with the Rays this year, but he exercised an opt-out clause in his contract due to his strong hitting in the minors with an .866 OPS.
Ben Nicholson-Smith of MLBTR points out that Shealy has shown the ability to hit major-league pitching and relays an excellent point of Rosenthal's: This has Mike Lowell insurance written all over it. If we move Lowell now, we immediately have a bench bat who can play 1B (Youk obviously can play 3B if Beltre were to get hurt).
Maybe he's reading too much into it -- Theo has a history of picking up reclamation projects like this (see Hermida, Jeremy, Hall, Bill, and about 4,000 other guys). But I think he's right: it's another sign that Mike's on his way out. If Shealy signed on after opting out of a minor-league deal, he must have some assurance that he'll be on the big-league roster.