I've seen 1" = exactly 25.4mm some places (I use that one at work since it's easy to guesstimate and we design in inches anyway.) I think it's one of the official versions of the inch.
According to the NIST cite, http://physics.nist.gov/Pubs/SP811/appenB.html (Scroll down to section B.6):
"The U. S. Metric Law of 1866 gave the relationship 1 m = 39.37 in (in is the unit symbol for the inch). From 1893 until 1959, the yard was defined as being exactly equal to (3600/3937) m, and thus the foot was defined as being exactly equal to (1200/3937) m.
"In 1959 the definition of the yard was changed to bring the U.S. yard and the yard used in other countries into agreement. Since then the yard has been defined as exactly equal to 0.9144 m, and thus the foot has been defined as exactly equal to 0.3048 m. At the same time it was decided that any data expressed in feet derived from geodetic surveys within the United States would continue to bear the relationship as defined in 1893, namely, 1 ft = (1200/ 3937) m (ft is the unit symbol for the foot). The name of this foot is "U.S. survey foot," while the name of the new foot defined in 1959 is "international foot." The two are related to each other through the expression 1 international foot = 0.999 998 U.S. survey foot exactly. "
BTW, 0.3048/12 x 1000 = 25.4 (eggzactly), so yer right!