ok, so I got curious and googled a bit - here's what I came up with:
nothing definitive on the actual NRMP site, but a few links below...
http://www.ama-assn.org/ama/pub/category/9781.html
and
from this link:
http://www.cordem.org/news/2003/jan-feb/nrmpchanges.pdf
The NRMP Board of Directors, at its November 2001 meeting passed the following motion:
?Beginning with the 2004 Match, all sponsoring institutions participating in the
NRMP Main Match must register and attempt to fill all their positions in the Match
except for those specialties or programs participating in other national matching
programs.?
The 2004 start date was established to permit the NRMP to work with institutions, program director
organizations, and the OPDA to develop a process to carry out this new directive in a fair and reasoned
manner. The new rule addresses the present inequality in NRMP procedures from the perspective of those
who are in control of the selection ? institutions and programs. Beginning in 2004, institutions and
programs participating in the Match will not be permitted to fill any of their positions outside the Match,
e.g., have some positions in the Match and fill other positions outside the Match. Therefore, programs
will not be permitted to fill positions with independent applicants outside the Match.