I bought a set of 27 x 1 1/4 Michelin World Tours for my Raleigh Sprite 27 a few weeks ago--it has pretty chunky 27 x 1 1/4s on there now, so figured they'd fit. They don't, though--there's less than a mm of clearance to the chainstay on each side, with a perfectly true wheel. Not enough to my way of thinking.
This has happened to me a time or two before. It's a little frustrating to have to buy and install a tire to find out whether it fits.
You know what would be a public service? A data base of widths of various tires on various rims. Those who want to help out could measure the fully inflated tire width with calipers whenever they mount a new set of tires, and send that information to the keeper of the data base (not me) along with the data for the rim. (How much bearing does the rim have on the tire width, anyway? Some, but not a whole lot, I'd guess.)

There are a lot of tires and rims out there, but there are a lot of us, too--I wouldn't think it would take too long to cover a lot of ground.

For what it's worth, a 27 x 1 1/4 Kenda Street tire (pretty crummy tires, by the way) are 31 mm wide when mounted on a Mavic G40 rim.

I will feel kind of dumb if such a data base already exists and everyone knows it but me. By the way, the World Tours are available for trade. They're essentially new--I went on one 40-50 mile ride with them.