There is a minority that put the Apple logo on them, which is clearly illegal, but...
Beyond that the look of Apple's products are protected by design patents and the law on them is much less clear cut and varies by country (see the Samsung case for example).
There are a large number of non-iPad knock offs available for under $125, though. A quick search on DealExtreme will help..
I'm pretty sure that even Apple isn't really concerned about a hand built, 5cm thick, Windows 7 running, rhinestone encrusted "tablet" being mistaken for an iPad.
I don't see how is a "knock-off" illegal. They try to mimic the real one, and it looks like crap, but it doesn't mean it's a illegal product. It's not like they're stealing anything.
Well, they're not really illegal in China as far as I understand, as long as they don't slap an Apple logo on it, having the same overall design is fine...