Just to follow up on this post, I just came back from the trip yesterday. A few notes:
1) The boat we chartered, Tallulah, from DSL Yachts, was by far the best equipped / nicest bareboat I've ever chartered, far better than what Moorings offers. It's a 46' Bahia catamaran with AC and all of the goodies. Because they're not Moorings, they charge about the price of a Moorings 4200 but it was a heck of a lot nicer. Having a watermaker on board was really nice and avoided us having to stop for water, particularly important in the very spread-out Grenadines. Though we never used it, this boat had a LCD TV / DVD player. It also had an iPod hook-up, top-of-the-line navigation equipment, and extremely nice sailing gear such as auto winches for trimming the sails without a lot of manual winching. If you're going to go to the Grenadines, consider getting Tallulah from DSL Yachts in Rodney Bay, St Lucia. The one downside of chartering from Rodney Bay is it's a far sail from the Tobago Cays (about 75 miles), whereas you can charter Moorings in Canouan (which is a ***** to get to though). We enjoyed the fun open-water sail both directions between St Lucia and St Vincent, though, which is about 30 miles if I remember with nice swells and 20 kt winds.
2) If you are a scuba diver, you will find that diving in the Grenadines is relatively expensive and it's not very practical to rent tanks and have your own gear because the Grenadines is pretty spread out and there aren't a lot of fill stations. Plus, some islands won't let you dive legally without a guide. There is a market called Seaside View on Petite Martinique, however, which rents tanks for the equivalent of about $4.50 per day which is unheard-of inexpensive for the Grenadines. It is really meant for renting tanks to local fisherman, and the guy who runs / owns it will have to get comfortable that you will not steal his tanks (he doesn't take a deposit or credit card or anything) but I found his tanks to be in very good condition and he was a nice guy. He doesn't want you to go far, though and you can only take the tanks out for the day. He opens at 9:00 and he usually wants the tanks back by 12:00 or 1:00, so you have to do local dives (e.g.: Mopion reef) and have your own gear. The Mopion dive is decent- I would give it a 5 on a 1-10 scale. We also dived in St Lucia and on Bequia, both with guides, and I enjoyed both. St Lucia was with Action Adventure Diving which was good, and Bequia was with Dive Bequia, also good.
3) If you charter in St Lucia and go to the Grenadines, you will have to clear in and out of customs four separate times (out of St Lucia, into the Grenadines, out of the Grenadines, and back into St Lucia). This is sort of a pain and takes about 1 hour each time. Also, there is some cost, but not huge (about $50 average each time). That's definitely one advantage of chartering in SVG if you're planning to sail SVG. However, St Lucia is much easier to get to from a flight perspective, with flights from American, Delta, and US Airways, so you'll probably save much more time flying in than you waste in customs & immigration.
4) Drag a fishing line / lure behind your bareboat, particularly for the open-water sails. I had done this before and not had that much luck, but we caught an 80 pound blue fin tuna on this trip and it fed us for the week (tuna sushi, tuna steaks, tuna burgers...).
5) Bring some oral antibiotics if you have extras or can get a prescription. I contracted food poisoning (my own fault- came from our cooking, not a restaurant- we left the food out) and ended up in an emergency room in St Lucia. They charged me $300 US just to see a doctor in order to get a prescription, and it took 4 hours.
Enjoy your trip- it's wonderful out there.