True story, I was involved with a group of people that tried. We had about 3 programmers and a professional poker player working on the project full time. Any poker client bot will recognize the cards by the picture, since there is no data dissemination. Therefore the bot needs to run on your computer to have access to a screen. The poker sites make you sign a waiver that allows them to search through your computer while you play. The poker client will look for known bot programs, as well as take screenshots of your computer looking for fishy software. To get around this we had to set up virtual machines, which wasn't that big of a deal. However, the poker site would still sometimes catch us and take the money in our account.
Second problem was the logic. This is much more difficult than it sounds. It is much easier to write limit hold em poker, than no limit. Even with limit poker though, the good players would figure out your bot's patterns, and take advantage of them. Therefore we needed to hire very low wage earners (I was doing this out of a third world country) to constantly move the bots around to new tables.
To go after the less sophisticated players, we attempted to play the lower limits, which required more human attention. Scalability was a big problem.
We were able to cover our costs and make a little money, but it was a very slow grind.
In a related story, I know a 1st generation Chinese American poker entrpreneur who started a poker sweatshop out of china. He hired an army of smart kids, taught them to play low risk poker, and pays out a % of profits to the players. He's making a killing.