Bikram Yoga is the method of 30 poses and they heat the room to 98 degrees. You can read more about it on the internet.
The reason I think Bikram helped me swim better more than anything else is because it opened up my upper body, my lungs, my reach, extension, flexibility etc ...... I can not tell you how much it rocked my world.
Bikram yoga is tough, I had no idea how tough it was I could not finish my first 5-6 classes I could not even hold my arms up at the the end of the class. I thought i was fit :) but there were people in there who looked not in shape just blowing me away.
Its all about the balance of controlling your strength. Try it but you have to give it 3-4 months. I would incorporate a run before the 90 MINUTE class !!
chris
As an alternative to Bikram there is Iyengar Yoga. No heated room the the practice focuses on precise alignment and balance. Iyengar has been recommended as being particularly good for runners/triathletes for the same reasons given by christri above.
Be it Bikram or Iyengar or Astanga, give yoga a try. If you stick with it and do the practises, you'll be amazed at the results.
I do bikram but would not recommend doing anything before it as bikram's will dehydrate you enough. You need to make sure your fluid intake is good enough throughout the day in order to finish, particularly if you have a high sweat rate. I have done ashtanga and power as well and all have been great for flexibility. I use bikrams to acclimitize to heat for hot races as well.