After reading all your very helpful responses, I think my plan is to sign up for the 2-day beginner course at the Mariner Sailing School (near DC) and spend a good part of this summer taking advantage of the cheaper boat rentals on the Potomac to practice. Thanks for reassuring me about wind conditions on the Potomac, that was my main concern about learning here, rather than on the Chesapeake.
Then next summer, perhaps I'll take one of the more advanced keelboat classes on the Chesapeake. Looks like it would be hard to go wrong among several recommended programs there, although I'm leaning slightly toward the 4-day live-aboard program at the Maryland School of Sailing.
Thanks again for all the great suggestions.
I learned the hard way with a very disappointing skulling class last year that good instruction is very important - particularly with regards to boat safety (not until I dumped myself in the Potomac catching a crab on the last day of class did I realize we had done no capsize drills at all - I actually had to be rescued from the water - what if it had happened the next day when I would have been in the boat alone?) I'm determined to do my research this time around.