In the Rob Antony Q&A at Over the Baggy (recommended read!) he tips his hand on Casilla
"When we go out and get an Orlando Hudson, we think Casilla could be an everyday second baseman. We just aren’t ready to bank on that right now. He hasn’t shown enough. If he had backed up last year like he had in 2008 then you might not do an Orlando Hudson. But for a year, now we’ll see if Casilla gets another look, another season to see if he’s ready to handle it. If not, we’ll have to go get somebody else."
That's a pretty strong confirmation that Casilla is making the roster.
The whole interview is very good: http://overthebaggy.blogspot.com/2010/03/twins-and-statistical-analysis.html