There were people who figured that inscription would pretty much die out by the time that the dual spec thing died down after 3.1. In my case, it has not been true at all. I still make at least a few hundred gold per day selling glyphs.
You can sell all kinds of glyphs, but the ones that sell the best seem to be Pally, DK, and even Hunter glyphs. I sell glyphs for all classes, but these are often times my big sellers.
Despite having an herbalist with epic flight, I choose to buy my herbs to mill. I have been getting many, many stacks of icethorn for under 17 gold per stack. I mill it and usually get at least one snowfall ink and 4-6 Ink of the Sea from a single stack. I use the ink of the sea to make glyphs and usually sell some of the snowfall ink on the AH for 25 gold each. It is not rare for me to end up with two snowfall ink out of a single stack.
With my Ink of the Sea, I browse through my glyphs and see what is selling well. I exchange Inks of the Sea for whatever ink is needed for the glyph and Im in business.
I have been trying to keep around 250 or so glyphs on the AH at any given time. I always undercut, but never go below 8ish gold on any glyph. If the prices are getting too low, I hold onto them for a few days until the prices go back up.
A nice little niche seems to be the minor glyph market. There usually seems to be a lot less of them flooding the AH and many will sell for over 20 gold each.
Stay away from the Glyphs that people use to level. They will be worthless. People sell glyphs such as Glyph of Sunder for under a gold. I dont get it. You may as well just vendor the damn things and be done with it.
Inscription is still going strong, and if you really try, there is no profession that wont make you a bit of cash.