First off, today is Just My Two Coppers' Blogger Carnival! Be sure to check out some of the other great WoW blogs and help us grow as a community. Hopefully you'll learn from our mistakes before they become your own!
I've been hanging out in JMTC's IRC channel lately. They have a webchat interface so folks can pop in and out as they please and since pre-Glyphmas there has been an increase in new faces appearing. Some are great new additions to the chat, some ask to be spoon-fed information that is already on the forums (pssssst... go read the forums!) and others ask for confirmation that a gold-making method will work.
My response to this last part is always: YMMV (Your Mileage May Vary)
There are some safe bets, like Saronite Ore for less than 62s50c per piece (smelted into Saronite Bars, a stack vendors for 25g). If Eternal Earth is less than 5g per piece and green-quality gems (Chalcedony, Sun Crystal, Bloodstone, and Huge Citrine) are less than 2g per piece, you can always craft necklaces and rings that vendor for a small amount of profit.
Everything else is really server-dependent. Sure, we can tell you that something worked great in our own experience but it may not necessarily work for you. Just like the United States and France have their own economies, so do Moon Guard and Bloodhoof. The only way to know how well something will work is to know your economy and, since many goblins are secretive about which server they play on, your chances of finding information tailored to your needs is not so great. You'll have to figure things out on your own but remember that with great rewards comes great risk.
If you want to start understanding your server's economy, look at what your server base is like. PvP servers love PvP gear and RP servers love shirts; servers with a lot of progression will need more flasks and servers with a lot of altoholics will need more bags. Be sure to do your daily (at least!) auction house scans if you're using Auctioneer. Watch how quickly different types of goods sell and be wary of those that languish for a long while. These are just guidelines, of course, but hopefully it will be a start.
Here's hoping you get great mileage to the (gold) gallon!