Ok now this is going to be a slightly boring post, as I just want to put somewhere up actual details of how to get to see the Amish, since most of the tourist books (including lonely planet), and the various operators seem to have no clue!
First off, Philadelphia is probably the nearest big city to the Amish (obviously they're not exactly city folk so a bit of travelling is required). When you get to Philly, you want to go to 30th St Station go to the Amtrak building and buy yourself a ticket to Lancaster which is on the line towards Harrisford it's about $13 one way.
When you get to the Amtrak station sit back and wait out the front (in the shade) and eventually (they run about every 15 minutes, and it's FREE!) a 'Trolley' will arrive, basically it's a bus built like a tram/street car. Catch this to Duke St bus stop (don't worry you'll recognise it).
Yes you read that correctly, you need to catch a number 13 bus to Intercourse it's a county out East (I think) from Lancaster (about 30 mins), and you will start to actually see some Amish related merchandise and shops. And there ya go, you'll be able to see some real live Amish folk wandering around, and you may of even caught the bus with some of them (aparently they can ride in automobiles, just can't drive them).
Just do the reverse, just do a bit more planning than we did, and not have to run to the train station because you've missed the last 'Trolley' and only arrive at the station with 20 seconds to spare!
That's more detailed instructions than I found in any of the tourist guides, they all had great info about the area, but no info on how to get there, other than to say 'Pennsylvania Dutch Country'... (where the F is that?!?)