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Hey Tailwag (what's your real name, btw?). It's not an uncommon question, and one that mojo has asked me many times "why aren't you using your 350D to take images of the planets?"
Yes, the 350D has a much bigger chip and much more resolution - 8mp (over 3000 px) as opposed to < 1mp for the webcam (640x480 resolution). However you can generally only take a few shots at a time in burst mode, 3fps at the most for a few seconds before it needs to write out to the card.
Compared to the DMK21AF04 webcam (yes, it's better than a $30 webcam), I'm doing 30fps for as long as I need to record. For the short exposures we're using, the webcam has a more sensitive chip and will pick up fainter detail during that exposure.
The stacking process we use in planetary imaging means we take all the "good/sharp" frames and stack them (to reduce noise) and throw away the bad frames. When you start with 1000+ frames at 30fps, you can afford to do this and you can capture those moments of steady seeing. You might end up stacking 100-300 of those.
At only 3fps (or less), you're more or less taking single shots which mean (on average) you have much less of capturing a frame during that steady seeing. You'll also end up with far less frames to stack to try and reduce noise.
I have seen some planetary images taken with a DSLR or digital camera, and some have been quite good, but I have never seen a DSLR shot beat the best webcam shots in the hands of the experts.
There's plenty of digital camera shots of the moon, and because it's so big it's great for those "whole moon" shots either at prime focus or afocal. However for closeup shots of the moon (individual craters or features at high resolution), then again the webcam will beat the almost DSLR every time.
The webcams we're using aren't $30 jobs - the ToUcam is the best of the budget range, at around $150 and is great in low light, fairly low noise and for the price, is a superb introductory webcam for planetary/lunar imaging. As you get more experience, you can spend much more ($600-$2000) on a webcam which has a more sensitive chip again, less noise, higher framerates, faster interface (no compression), larger format chip, more dynamic range (12 or 16bit as opposed to 8bit).
Hope i've helped a bit.
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