I believe that the Rangers have only signed two Cuban defector in their history, and those two -- Osmani Garcia and Jorge "Spider" Diaz -- were back in February, 2000.*
* Only signed two as the original signing team, anyway.
That was a long time ago...the Rangers were coming off their second sweep in as many years against the Yankees in the playoffs, and Rangers fans were excited about the acquisition of lefty starter Justin Thompson, who was coming off of shoulder surgery but was expected to be ready to contribute by early in the year, and newly signed free agent starting pitcher Darren Oliver.
In any case, John Manuel of BA had a write-up on Martin when he defected last year, saying that Martin was expected to command a seven figure bonus. Manuel says he's a speed and defense guy, with "at least" average range in centerfield and a propensity for drawing walks in Cuba.
It would reportedly be a major league deal, meaning Martin would immediately go on the 40 man roster.
My initial guess is that Martin would start off at high-A Myrtle Beach. Engel Beltre is holding down center in AA, and I don't think they'd immediately thrust Martin into AAA, although he could move quickly, given his age, if he shows he can handle it.