Besides, given the weather this year, you're apt to be on dry trail well past the 7,000-foot mark, certainly as far as Pebble Creek, a popular place to stop for a drink and a key landmark between Muir and the Paradise Inn area (as you know, the start point for the climb proper). There is an issue of ankle stability with what is apt to be a large load, but that's for you to decide. If you think you'll be OK with light shoes, then go for it. The trail up to the year-round snowline is broad and smooth, with cut-stone stairs in many places. On a warm day you may even be happy wearing light shoes all the way to 10,000 feet, but if it's cold and blustery, or even raining, then you may opt to don the plastics.
As for which shoes, I'd say any low-cut day hiker would suffice. I would, however, avoid a trail runner, because the soft midsoles may lead to bruised feet. Get a light hiker that's a cut-down version of a true boot. The Garmont Nagevi XCR ($115; www.garmontusa.com) has proven to be an extremely popular choice in this category. It even has a Gore-Tex bootie, although that's pretty useless the instant you overtop them when hopping a stream. But they're grippy and stable, and may get you all the way to Muir. The Asolo Axis ($85; www.asolo.com) is another good low-top hiker, with Vibram soles for good grip. And check out the Five Ten Via Ferrata ($115; www.fiveten.com), which is both waterproof and has a pretty stiff midsole.
Hope you have a great climb! If you haven't been up Rainier before, it's a wonderful experience. Well, except for the long, baking slog up the Muir Snowfield, the midnight start, and that long, awful, mind-numbing descent...
Check out more über-footwear in Outside's 2004 Buyer's Guide