T.R.
I have had 2 bouts of PF. Once was in only 1 foot, lasted a few months, got inserts, went away.
NO ONE TOLD ME that I had to keep up with stretching/appropriate footwear/etc. when it wasn't hurting.
So it came back, both feet, lasted just over a year. I required physical therapy & special inserts, & a special sock @ night to keep my foot stretched while I slept.
So, if you have had it once before, you are now more prone to it forevermore (per my foot doc). Yoga seems to me that it would encourage stretching of your body, & also the use of some of the muscles in your feet that you don't normally use in those ways, for stability in the poses.
Now, a quick search online pulls up a lot of sites on yoga stretches to HELP PF. I would first of all talk with your instructor about what they know you can do to alleviate the problem. If they DON'T know, or aren't aware of anything, then find another instructor. No point in doing something that's supposed to be good for you, if the person leading you in that activity is not knowledgeable.
Secondly, you need to make changes to your non-yoga time. Always wear shoes with arch support (Crocs are recommended as "house shoes" for their cushioning). Make sure to do proper stretches in the morning before getting out of bed, & then throughout the day as needed.
Stretchess:
Stand with the ball of your foot on the first step of stairs, heels hanging over, & sink your weight into your heels, letting them drop below the stair surface. This will stretch the back of your calves down through your heel.
Cross one ankle over your knee, grasp your heel in one hand & the toes in your other & push/pull to stretch the bottom of your foot. This link has an example (I add cupping the heel w/your other hand & pushing on your heel while pulling on your toes/ball)
http://sportsmedicine.about.com/b/2007/01/05/new-stretch-...
If you find that this is not helping to keep PF at bay, you can also get a Strassburg sock. This is something worn @ night to keep your foot flexed. (If you get one, be warned, using the proper tension will cause your toes to have a stabbing pain in the morning until you take it off, this only lasts a few days until you get used to it)
Hopefully a good yoga instructor, stretches, proper foot gear & maybe a sock will help keep the PF at bay! (I've found your feet tell you when they are about to get worse by feeling really "tight" before throwing out a lot of pain. Once the pain starts, you're in trouble. Back off @ tightness & then resume slowly)
T.
Edited to add: I noticed another person commented on bare feet/hard floors. If possible, wear some type of foot gear doing yoga & see if that helps. Try to find something that cushions/supports to some extent but still allows flexibility. Ask your instructor for advice.