Speaking as someone who does marketing for a living and has a master's degree in it, I would say my assessment would be that this will make people think twice about prices of tablets.
The problem with consumer psychology is that a big sale makes people want to wait for the next one. Once people know "what's possible" by waiting they are less apt to buy something at full price unless they really need it (and let's face it, tablets are a total "nice to have" product and not a "must have" product). This is going to affect the iPad knock off market more than Apple, though.
As far as iPad sales, they will not be affected much because Apple is perceived as a premium brand and so a high price is expected (and in fact necessary to upkeep the premium image; if Apple were to drop iPads to $299 or $349 they may very well lose their premium image over time).
I think it will also make companies think twice about their pricing and positioning. Apple competitors now know how to compete with Apple on market share. Seeing HP sell hundreds of thousands of units in one weekend tells them that if they price their tablets much lower than the iPad they can get market share.
However, the key is that the tablets still need to be decent. The reason $99 or $149 tablets aren't selling now is because they are crap (to get to that price point their bill of materials must have been $75 or so).
The HP TouchPad is actually a much higher quality than those. Now a $99 price point is not sustainable for the Touchpad given its BOM being around $280-$300 or so, but a $299 or $349 price point would have helped a lot in generating sales.
I think tablets priced around $299 or $349 but with the same quality as the Xoom, TouchPad, Galaxy, etc. would sell better, plus they would be reviewed better.
The main reason HP's touchpad was not reviewed well is because as a $499 tablet it is just not as good as the iPad which is priced the same. However, if HP priced it at $299 or $349 from the get go, reviewers would have cut it a lot more slack and better reviews would have led to more sales.
I think a $299 price would actually be a sweet spot since $299 would be perceived psychologically as MUCH cheaper than $3XX.
So no, I don't think this has HURT the market, I think it EDUCATED the tablet market (both the consumer and manufacturer sides)