Honestly the truth is that there is no right or wrong answer to this question. What your goals are when you play is what will determine which path you should take.
If you actually enjoy the process of making gold (as opposed to just having it); view gold making as a part of the end game content; and possibly have dreams of hitting gold cap then you should diversify. Generally this would mean having more than one toon with max professions (or better yet all maxed professions) and posting in every market when it was profitable. This is going to take a bit of time (especially leveling all the toons and professions) and honestly isn't for everyone. You are guaranteed profits over the long haul as when one market declines you can just shift resources into another more profitable market but there will also be times when you list items at a less than perfect price due to the sheer number of items you will be listing on the AH at any one time. The fact that using this method guarantess profits over the longhaul along with the ability to take advantage of any gold making opportunites offered by patches/profession changes means that this is the method of choice for people who enjoy the process and are in it for the long haul. People who are just interested in getting a bit more money to buy a chopper or Sandstone Drake, however, are probably better served with specializing in one market.
Specializing in one market won't bring in the same amount of gold over the longterm when compared with diversifying, but it won't take up nearly as much time either. If you get enjoyment in Warcraft from spending all of your time PVPing, raiding, and/or achievement whoring the decreased time commitment of this strategy is probably appealing. One toon is all you need (maxing both professions will definitely help) and you will be able to learn the ins and outs of your market much, much quicker. Using this method you should be able to quickly see your total gold rise and buying any item you want in game will be quite possible. You might not be able to take advantage of every gold making opportunity created by professions/item changes - but what do you care? You're having fun raiding with your guild - not being stuck at the AH.
Enjoy making gold - diversify. Enjoy having enough gold to buy what you want in game - specialize. Somewhere in between? Then just choose a few markets to diversify into as opposed to taking part in all of them.
As a side note for anyone interested in Rift I have just started a new blog here. I am talking about the game basics now but will be switching into how to make platinum in the coming weeks.
Have fun making gold!