There may be a requirement by your program but there's no ABIM rule about it. One of the fellows who graduated from our program last year took the IM and Hem/Onc boards back-to-back after finishing up the program.
I missed the deadline for applying for the IM boards this year (at least, without the late fee) so I'm putting it off until 2010. I'll probably have to study a little more but, because of being in the research pathway, even if I took it this year, I'm already 1y out from general IM and since the IM board exam is, from what I've heard, basically the rheumatology boards with a few cards and heme questions thrown in, I'd need to study anyway.