Travis, I grow tea. So far my plants are small, but you definatly need cold hardy varieties for Z 6. Camellia Forest Nursery is a great place to buy cold hardy plants or seed. The small leaf Japanese variety is cold hardy, and they have others that may survive your zone. They really got to test cold hardiness this year, so perhaps they will have other suggestions. The Sochi tea sold by them and Burnt Ridge should be cold hardy, it certainly is one of the most northern grown teas of the world. My tea plants are from various places, and many were killed to the ground this winter (Z 7). A heavy mulch and the proper site can help with survival. If you see folks growing Camellias in your area, then you are likely in the right zone for some variety. Also, they like acid, well drained soil.
I love all forms of tea. I drink it all day, mostly green tea. There are so many variations, flavors and different preperation techniques that I could never get bored. You will often hear that all tea is made from the same plant, and this is partially true, but just like other plants bred for certain purposes, some varieties work better for certain teas. In general though, you can produce green, white, black, yellow, oolong and Pu'er from any tea plant, but may not get the same flavors or quality. I like to taste the new growth on my plants in the spring to see what flavors are there, and my goal is to find tasty, cold hardy varieties for my area. Making tea can be labor and space intensive, but I think many homeowners can grow enough for the families consumption. Many small tea farms are now in the US. The farm in SC is the longest survivor, and others have tried and failed. Hawaii probably has the best chance of producing high quality tea, as many select varieties adapt there well. Im sure in time high quality varieties will be selected for growning in the continental US. Right now, it is in its infancy, so like figs, it will take time to find out what survives where, and what tastes good. There is also the problem of labor in the US. The best teas are hand picked, but there are now machines that gently pluck the new tea buds, so perhaps it can become commercially feasible.
If your interested in other "tea" with caffeine you can grow, you might also look into Yaupon holly, a US cousin of yerba mate and guayusa. I think it survives to Z 6, and makes a good, healthy drink, and very easy to grow.