The problem of the "arbitration" clock argument that people espouse about prospects is that, outside the top 1/6 of players with less than 3 years experience, DY will still not be eligible for arbitration until he's been there 3 years - and that shouldn't change whether he's brought up now or in September.
Bringing DY up in September will not speed up his arbitration clock anymore than if he started 2006 in TB since he's already behind all of the rookies who have started their clock this year and will be ahead of him such that he doesn't fall into that 1/6. Heck, the Braves alone have at least 8 players whose clock is ahead of DY (Davies, Boyer, Betemit, Johnson, Francoeur, Langerhans, McCann, Orr, maybe Colon if he stays with Detroit, Marte if he's called up in September, etc.). So, if TB plans on playing him from the start in 2006, then there'd be no reason not to call him up in September - especially since they talk about what is best for his development, yet what works better for his development - playing baseball the final 3 weeks of the season or watching from home when the minor league season ends in early September.
Now, if the Rays don't plan on him being in TB at the beginning of 2006, then this would be a decision not to start his clock and I could see them not calling him up in September. However, to do so would further annoy fans (like me) who would like to see the future of the franchise.
So, based on the number of rookies in 2005, bringing up DY now should still not make him arb-eligible until after the 2008 season. If the Rays are going to try and push that until 2009, then he won't be brought up in September, nor would he likely start 2006 in the majors. As I just can't see the latter happening (or at least I hope it doesn't happen since I'd like to see him play), I fully expect that DY will make it to the majors this year.
With all of that said, I agree with raiders, though. Take the bird in the hand. It isn't as if Felix stinks. He's a great pitching prospect. And while it won't affect DY's ability to go to arbitration before the end of 2008 if he were brought up now, it would not surprise me that the cheapness of the Rays would cause them to keep DY in the minors through April of 2006 to push that until the end of the 2009 season. While I honestly don't think they will, it is possible, so take the sure thing with Felix.