"Playing" the auction house has been a fun diversion for me too. I started off with about 100g on my enchanter / leatherworking toon a couple of weeks ago and I'm up to 5000g on him without any other source of gold; just the AH.

Just to further the comments already made, you really have to pay attention to the market you are in. DE'ing greens and reselling the mats has been a slow but steady income in the AH for me but you have to know which level greens and when to sell. I imagine it varies greatly depending on servers.

I stay away from buying and reselling epics. I was able to do ok on a couple of transactions but it tied up my money for far too long. Turnover is the real key here. Moving your sales as quickly as possible so you can reinvest is what I learned to be the best catalyst for increasing my output.

I wouldn't ever advise anyone to try to corner a market; I'm sure there are plenty of people that have been able to make it work but you really have to hit a particular market at just the right time and that involves a lot of observation and time. It could work really well but like the guy on my server that tried buy out all the frozen orbs, it could end up costing you big. WoW is different than the real world in that commodities are of unlimited supply.

All and all though, I probably spend more time playing the AH, watching markets, performing scans and what not than it would take me to go do a full days worth of dailies. The benefit in my mind is that it is something different. It's an old cliche' but diversification of income sources really is the name of the game.