Yeah, glass covers are the way to go. They're tight-fitting so they really help with evaporation control, plus the higher-wattage lights designed for plant growth can get pretty hot. The glass can withstand these temperatures, but I wouldn't feel comfortable using those lights over plastic.
Also: are you guys talking about the Current compact fluorescent fixture with the lunar lights? They look great if you ask me, and are definitely well-priced. The only problem is that they come with a 50/50 bulb, which is designed for saltwater use. If you were really nice, they might switch out the bulbs for you to something more suited to freshwater.
Aquariumguys sells the same fixture, but their normal price is about the same as Drs. Foster and Smith's sale price. Here's a link to the 24" version: http://www.aquariumguys.com/24satellite.html
Little-Fizz, on a standard 20g tank I think that would provide some really great lighting for growing just about any plants, provided you got them to replace that bulb for you.
Here's the right kind of bulb: http://www.drsfostersmith.com/produc...4&pcatid=13314
You want the 22", 65 watt Dual Daylight SunPaq that's 6700K/10,000K. The aquariumguys website didn't seem to have them for sale, so it might be better to get the fixture from Drs. Foster and Smith since they can probably switch out the bulbs for you.