Strictly speaking it's true....with a but!
If we're talking about the focal length as printed on the lens then it's not true. Due to the way lenses with internal focus work then the real focal length will normally decrease as the focal plane moves backwards from infinity. Differrent internal mechanisms will result in different "true" focal lengths at specific distances.....ie, an 85/1.2 will very likely have a different "true" focal length to an 85/1.8 when focussed in the same place....this will result in different magnifications.
Bob
Internal focusing is certainly a factor, though it is also true that quoted focal length only holds good for infinity focus anyway (like f/number). I believe that all lenses technically reduce focal length as they are focused closer, of any design, although the difference is fractional, like a couple of percent.
Internal focusing commonly often increases this to 10% or even more. A lot more in some cases - I've seen images taken with a Nikon 18-200 that even at quite modestly close distances were so far away from equivalent shots taken with a prime that you'd think they'd get done under the trades descriptions act.
I also believe this is the reason why some internal focusing macros when focused down to 1:1 do not lose the full two stops of light that theory (inverse square law) dictates that they should. What happens is that focal length is reduced to such an extent that the aperture becomes effectively larger and f/number is correspondingly reduced to the extent that some designs only show a reduction of 1.5 stops at 1:1.
And then again, the quoted focal length is moveable feast anyway, even at infinity. I'm not sure what the acceptable 'tolerance' is but manufacturers tend to keep very quiet about it - 300mm marked that turns out to be 270mm actual is quite common with some 'economy' designs. Economy of truth as well as cost.
All of which is a bit off the point of the thread, but it's yet another factor in the rather hit and miss application of depth of field calculations. The theory of it all is absolute, as it has to be, but is based on a whole series of asumptions that almost always don't fit very closely at all with what actually happens in practise, or with all the other significant changes that happen when you use a different lens, or change the shooting distance, or do almost anything different.
And here's another thing I've just remembered which is really anoying when working at close distance. Depth of field calculations, like those on
www.DoFMaster.com take the distance measurement from the front element of the lens, whereas the focusing distances marked on the lens itself are taken from the focal plane. There can be a foot difference between the two, and that's a heck of a lot when you're close
but its bugging me and i can't think how else to explain it
/
now my next question then is why does a shorter focal length give more in focus for the same aperture and subject distance than a longer lens 
