I replaced the 12-25 on my cross check (Tiagra RD) with a 11-32, and it works just fine. If I remember right, the guy at the LBS seemed to think 11-34 would be too big, but they didn't have one lying around to try, so I'm not sure.
Edit: looking at the tech documents, you are right that the small sprocket on the list is 11T, and max 27T. But the total capacity is 37 teeth on the long cage and 31 on the short cage. I've got the long cage. For my bike thats 12 (48 - 36, teeth range on the front double) plus 21 (32 - 11), or 33, well below the total capacity. If I had a 24 tooth granny gear for a triple front, I would exceed the total capacity though. If I had a 11-34 cassette, I'd still be okay probably.
Not a bike mechanic, so not sure if this is 100% accurate. But the answer apparently is: depends on the teeth difference in your crank and if you have the short cage or medium/long cage version.
Last edited by Quel; 06-25-09 at 02:05 PM.