HerbalisimFollow

I tend to look at any money that I get from Herbalism as "extra pocket change", instead of seeing it as a real money-making profession. It's really great because you don't use the herbs for anything but sales, but my real gold comes from fishing. The plants sell well enough, but the time spent in collecting them is better utilized elsewhere. I'm not saying that I don't make money from herbs - it's just that I don't go out of my way to farm them. I will head for any yellow dot I see on my mini-map if I'm questing in an area, but I make no special trips just to collect flowers.

As you level, fadeleaf is great for many rogues who do not have herbalism to make blinding powder.

In many servers there is no money in the low-level herbs though, except for Swifthistle and Fadeleaf.
I'd say that starting from Gromsblood it really starts to get interesting money-wise.
Blindweed is easy to get in large quantities and sells well.
Mountain Silversage is quite rare but sells very well indeed because of the use in some of the most popular potions.
Plaguebloom and Dreamfoil are steady sellers as any alchemist will always have need for these in large quantities (wasn't true for Dreamfoil but in the TBC era it really shines)
Major moneymakers are ofc Ghost Mushrooms and Black Lotus.
For Ghost Mushrooms you either have to run (part of) Mara, make your way up the big Troll city in the Hinterlands or fight your way trough a cave of slimes in the Hinterlands. Not easy to get, but tremendous payoff.
Black Lotus is just very, very rare.

With TBC once you venture into outland the name of the game will be initially the awesome new ability to "skin" certain types of mobs with herbalism, giving you motes and herbs.
As those motes are used in all professions for 300+ stuff you're all set to sail on the ship of money.

Dude, no way! NEVER sell herbs to vendors! In the AH you would be getting GOLD for selling Stranglekelp! :P

Really depends on the server. I'm also on a new server, still less than 6 months old, and I also noticed that briarthorn sells better than fadeleaf and swiftthistle. Actually, I'd be bold and say that fadeleaf and swiftthistle don't sell at all on my server. I get them returned to me from the AH over and over again, even though I'm the cheapest listed. Like Mike, I don't go out of my way herbing, unless it's for potions I need. So I haven't listed the rogue herbs for a while and maybe the market has improved. And dreamfoil, yes, I know they are used in all the protection potions, but they don't move on my server either. I've got 5 listed for 3-4 days now, again the cheapest. I'll give up after a week and make some potions and drink them myself. Don't listen to people saying that herbing and skinning makes money. It makes peanuts on some server, you need to take up the trade to understand the market. If you rely on them for your mount money, you might have to wait till you are level 60! My hubby skinner had light, medium and heavy leather returning to him most of the time before he gave up and picked up alchemy instead. Full stack of leather was selling at 20-50s at the time, really a waste of time for anyone in their 30s. Actually so was herbalism without alchemy. However, we find a gold mine in disenchanting. I'd wager our not-so-rich rogues are more willing to spend gold on +100 health, fiery, icy than consumables like fadeleaf and swiftthistle.