(Note: if you're looking for a definitive answer here, I don't have one. I didn't crunch the numbers or anything like that, this is basically a "thought" piece more than a serious analysis or suggestion. Keep this in mind.)
The conquest for "an ace" has lead many teams astray. Recent history gives us the 2008 Mariners as a team that ignored wounds elsewhere on the roster in favor of shipping an assortment of young talent to another club for a marketable frontline pitcher. The trade is poor even if Erik Bedard pitched at a high caliber level. Any team penciling Jose Vidro in at DH should probably focus on other aspects of their ballclub first.
Why then is the possibility of Felix Hernandez being acquirable so engaging? Because the Rays have few other holes to fill. Look at the roster and find holes. You find catcher and bullpen. Otherwise, the collection is about as solid as you can find. The Rays' window is open and should be open for at least another few years, but flags do fly forever and there are no guarantees that most if any of the prospects work out.
Acquiring Hernandez is more or less a rental. He's going to A) desire big money when his free agency clock hits zero and B) receive big money when his team control expires after the 2011 season. Can the Mariners afford him? Probably. Handing out big deals to pitchers has burned that organization enough that you can't blame the new regime - full of competent and intelligent people - to consider cashing in the Hernandez chips early if a home run offer is made.
Honestly, I'd put the odds of the Rays acquiring Felix on the low end of the pole.
The package in return would scalp the system, and cause a shifting dynamic in the competitive window. Imagine an actual window. Okay, now adjust that window on a five-year scale, where every few inches represent a season. Right now, the Rays have a nice collection of talent waves, so much so that you would figure they can field some pretty nice teams over the next three-to-four years. After which things become a little murky at this point in time. This means the window is more than halfway open. Okay now imagine the Rays trading three of their seven best prospects and someone like Jeff Niemann or Matt Garza to the Mariners.
The window doesn't slam closed or anything, but you put an emphasis on the next two seasons; after which Felix leaves, you get draft picks, and you are without the services of Desmond Jennings or Wade Davis or Jeremy Hellickson, whomever. On the flip side you get Felix, one of the best pitchers in the A.L. this season and probably the runner-up for the Cy Young award. He's always had the potential to be a special pitcher, and if this season is just a sign of things to come there could be some historic seasons thrown in along the way.
Depending on your perspective - long-run versus short-run or "win now" versus "win now and later" - you can make the case for or against acquiring Hernandez. I can guess how much it would take to acquire him, but I would expect - and I suppose that sort of ruins the concept of having this on a wishlist*, but whatever - Andrew Friedman calls Jack Zduriencik at least a few times about his prized ace.
Isuppose I'm just lukewarm on "going all in". I enjoy and desire winning, but at the same time you're talking about putting a ton of pressure on everything going just right within these next 324 regular season games while sacrificing a lot of contributors in the 324 that follow.
Some other thinking points to consider:
1. The inherent injury risk with all pitchers.
2. The value of marginal wins.
3. The value of keeping him away from Boston/New York.
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*Also: isn't saying "Well, I would like Felix, but I'm sort of hesitant to part with what it would take to acquire him" going against everything that a wishlist constitutes? I really should've chosen a better name for this series**.
**Seriously. This is stupid.