What is song from TrueBlood when Bill & Sookie are together. The cello solo?

The song you are referring to is by the show's composer, Nathan Barr. There will be a soundtrack release in the not too distant future which will feature (among other compositions) Bill and Sookie's love theme. You can check out Nathan's myspace page at www.myspace.com/nathanbarrmusic or his website nathanbarr.com to learn more about the composer and his work.