Fred Thomson has a low probability of winning enough delegates to be nominated as the Republican candidate.
He does have a good chance of being #2 on the ticket, especially with Romney heading the ticket. With the economy expected to go into a recession, Romney's economic experience and background will prove to be an asset. It will also allow him to focus on the economy and less on social issues. (Very similar to what George Bush did in 2000).
Being a southerner, with Senate experience, name recognition and someone who appears to be very congenial, Thomson can complement the Republican ticket with Romney.
In order to message this intention he should use his campaign resources to attack McCain & Huckabee when possible. By doing this Thomson can avoid having to explicitly make an agreement with the Romney camp (so that his attacks can be credible and sway voters) helping Romney win, especially, in those southern states where the Romney camp cannot spend their scarce resources.
A Romney-Thomson alliance can effectively battle the McCain-Huckabee forces and produce a ticket that the Republicans can back.