Some more info to reference:
http://www.valvoline.com/carcare/art...=ccr20010101ov

http://www.howstuffworks.com/question164.htm

http://www.noria.com/dictionary/
(for general reference and info)

The key thing to remember is that a viscosity number means nothing with a temperature at which the viscosity was measured.
In the case of 10W30
the 10 is the viscosity as measured at 0 deg C (32F)
the 30 is the viscosity as measured at 100 deg C (212F)

If you look at a UOA you will see that the viscosity is usually shown in cst at 100 deg C.

In all of the UOA's I've had done, the 10W30 usually return a vsicosity at 100 deg C of about 30-35.

edit: To Jim's comment on 10W40, I would not use it. With the lighter base stock there's more VI improvers (polymers) needed. These polymers do not provide lubrication - they simply keep the viscosity of the lubricant (oil) higher at temp. There's more to it, but there's no reason to use 10W40 with all the other options out there.