Refillable glass bottles are ideal if you have a local dairy that uses them. And classic! I'm fortunate to live in an area where they deliver, which is always fun just because it's seems so... historic!
I know I'm late to the party here, but I've recently discovered that my Whole Foods carries milk from a local dairy that uses refillable glass bottles. I haven't tried it yet, but I apparently I can buy the milk, and then return the bottles to WF for a refund. Sweet!
Have you ever tried to locate a farmer near you? You can search on RealMilk: http://www.realmilk.com/real-milk-finder/ - you also might be able to find one if you search for your local chapter of the Weston A. Price Foundation - those folks would love to hook you up with a local farmer or even lead you to a group buying club (which hooks the farmer up directly to the consumer.)
If you find a farmer and he/she offers milk in plastic only, you could suggest switching to glass or asking if he'd at least fill your bottles for you.
My farmer sells his milk in 1 gallon jars that he takes from the local deli and I just return them for reuse.