Since last week's patch, herbalists and inscribers have made a killing on the Auction House. In the long run, I find it it unlikely it will be an extremely lucrative market. Having had a week to look over inscription, I have to admit I've been very unsure about it. Initially I planned to teach my Death Knight Inscription and Herbalism, because I felt it would be as vital (or at least as profitable) as Jewelcrafting, but I find that to be optimistic at best. Most of the rogue glyphs have not really caught my imagination, although I did manage to pick up the the ones for Eviscerate and Sinister Strike (yes, I play my rogue just like I did at level 1) for a very reasonable 40g. At level 80, I have a chance to add one more major Glyph.
Most of you can probably already guess what the main problem is; once you've picked your 6 Glyphs, you have little motivation to switch over. Sure there will be upgrades as new patches are released, or factions introduced. However, the fact they are not directly linked to gear means players will not be seeking out a new Glyph every time they upgrade gear in an instance or raid. Currently, most regular players will see a steady stream of new gear from raiding, instancing, or playing in battlegrounds and arenas. For the vast majority of players, there is usually a better item around the corner which helps push the market for jewels, enchants and leatherworking armor. On the other hand, your glyphs simply scale with your character or gear, and there is no need for an upgrade until new ones are introduced into the game (which will be far more staggered than anything gear related).
So where does this leave players who have taken up the new profession? Of course there will always be some money to be made from any profession, and this one is no exception. So far I've heard from friends that many of the minor glyphs have a cooldown of 20 hours to learn, so even the most dedicated player will have to spend weeks if not months to learn all those available. Any sharp player will be very careful with the Glyphs he chooses to learn; common sense suggests picking the ones most likely to make money on the market.
This leads me to think that learning the glyphs for hybrid classes might be the way to go. A druid with healing glyphs will surely be forced into changing them if he decides to respec. So I would suggest learning Druid, Paladin and Shaman glyphs first, and in that order. My anecdotal evidence suggests that my druid friends are often most likely to switch spec. With the changes to Retribution, I also think many Paladin's with be chopping and changing between the talent trees, especially in the early days.
I admit I don't have much hands on experience with Inscription and all of these are assumptions I have made from the sources available to me. If the profession really is lackluster and fails to capture the player bases' attention (worst case scenario), Blizzard will be fast to make some positive changes; I don't see them giving up on a new profession as much as they have on engineering. Most likely it will be moderate success, with some profits to keep the inscribers happy, but not up to the levels of Jewelcrafting. Certainly the real winners here are herbalists, they've added another profession that depends on them, and have finally caught up with mining and skinning.
I'm not sure what professions I will give my Death Knight now. On the one hand, having inscription will make it easier to sell my enchants on the auction house, but my enchanter is on a different account I'm currently not paying for. Picking up herbalism will bring in some money and skinning could be a nice way to support my leatherworker (as well as make money). I think most people with only one level 70 will be facing a similar dilemma come November 13th; it's not everyday we get the luxury of being able to train two professions to a relatively high level.