No, it doesn't have to be pot bound in a one gallon pot.

What type of soil do you have and what area of the country are you located?

Don't know the soil pH (definitely not clay) right off hand but I have compost that I mix in with the soil before I do any planting.
I am in central NY (zone 5).

Janet was asking about the type of soil, I believe, not the pH since clematis can live in either acidic or somewhat alkaline soil. Is your soil free draining, mucky, does it drain well with added organic matter, etc. ? Clematis like moisture but they don't like to sit in standing water.

I don't believe anyone here has ever suggested letting clematis get rootbound in one gallon pots. Some of us do believe that small sized clematis are better served being potted up in one gallon pot so that their root systems get better established and more substantial prior to being planted in the garden however. This is especially true for very small clematis plants, especially the type IIs, since doing otherwise can make the plant more prone to attack of clematis wilt.

Soil here drains well...no muckiness.

My comment re: rootbound in a 1-gallon container...not sure exactly where I read about that as I've been reading in a bunch of different places/websites.

I planted my clematis this morning. When I squeezed the size of it's container it came out in piece (roots/dirt) and then the dirt fell away leaving me w/ basically the plant and roots. Hope is survives. I planted according to directions that came w/ the plant.

SDPA, could be that the soil in the container was too dry. I always water mine well and let them sit in the shade for an hour or so after soaking the rootball to allow the plant to take up as much water as possible. Some people submerge their pots of clematis in a bucket of water for 30 to 60 minutes to accomplish the same thing.