There is no reason to believe that gravitation propagates via quantized carriers. Physics' fundamental forces other than gravitation are uniformly mediated by vector bosons. Gravitons as tensor bosons satisfy the math but cause incredible trouble for creating a testable physical model, e.g.,

http://arXiv.org/abs/gr-qc/0409089
Spin-2 gravitons have problems
http://groups-beta.google.com/group/sci.physics.strings/msg/ba31a00f5f26277a
(so does the preceding proposal)

Promoting virtual particles to reality is the simple proof of their force mediation. Shouldn't that be quite facile for a massless graviton (e.g., making photons)?

Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/qz3.pdf