If McMurray can't come in for 2006, I don't see RR letting KB loose UNLESS:
(a) His bridges back to Rousch are on fire.
(b) They can land a sponser-friendly Big Name to allow them to jettison the reigning cup champ (only available one I can think of: Mikey).
(c) They want to reduce car count if sponsors continue to be difficult to keep.
Rousch Racing has had little difficulty in getting/keeping good "name" drivers, but this presents the chance to lose 2 this year (Martin/Busch).