Re: Why isn't mass considered a dimension?

The dimensions of a particle dimension can change without limits. Its rest 
mass cannot change, and its inertia can be calculated from its velocity 
using special relativity ( = rest-mass / (sqrt(1-(v*v / c*c))) ).
Perhaps you are confusing the physics definition of a dimension (roughly, 
anything which is a vector quantity operates at angles with respect to 
axes, where the axes represent a dimension) with the way the word is 
commonly used, e.g. "A box of fixed dimensions."
Also, in general relativity, mass (or energy) 'bends' 4-Dimensional space-
time, but is not a dimension in its own right.
In the 'Standard Model' of particle physics, mass is analogous to charge. 
Just as charge can be related to he cross-section (probability) of an 
interaction with a photon, mass is related to the probability of an 
interaction with the Higgs Boson. Thus, the heavier something is, the more 
Higgs bosons it will interact with, impeding its acceleration. There are 
suggestions that the Higgs particle has been discovered at the LEP in 
CERN. If not, we will have to wait until the LHC opens in a few years. If 
no Higgs are seen, our understanding of particle physics will need a 
radical overhaul.
This description of mass does not apply to its gravitational properties as 
no correct Quantum-gravity theory has yet been developed. Also, we so far 
have no way of predicting masses of particles, and can only hope that they 
will pop out of a Quantum-Gravity theory.
Enter String Theory.
The answer to your question about mass being one of the 10 dimensions of 
string theory is that it is not. String theory predicts 10 spatial and 
temporal dimensions (9 spatial, one for time). Six of them are, very 
conveniently, curled up nice and small so that they can't be seen.