I think the shimano 600 rear derailleur is rated to a 28t rear, though you can usually get a 30t into there. If you went to a 12-28 or 12-30 cassette, you'd widen your ratios a bit, but you'd get a 'lower low' gear, at only the cost of a cassette.

In fact, for 20$, nashbar sells a 12-28 cassette: http://www.nashbar.com/profile.cfm?c...eid=&pagename=

Ignore the 'ATB' name, a cassette is a cassette, and its only the ranges and ratios and marketing that differentiate between road and mtb. Its hyperglide compatible, and would likely bolt right on. The only adjustment you may need to make is the B tension screw, to move the jockey pulley down a bit farther to make room for the larger cog.

peace,
sam