I am currently getting AA miles from MCI, and would like to switch to another plan to get the signup bonus. What are my best options out there. I currently have miles on DL, AA, US, NW & CO in that order and would consider putting miles in any of these knowing that I can always switch back to MCI whenever the new plan expires.

Does anyone offer base miles as part of the signup deal?

Also, does MCI make you wait a certain period of time before you can signup again?