You can define shortcuts for notebook cell styles in the stylesheet editor, by adding
MenuCommandKey->"7", etc. to the style definition. That is, you can define Alt-1 to set/create a cell with one style, Alt-2 a different style, ..., through Alt-9.
From the documentation in Mathematica 8 for
MenuCommandKey for styles:
Acceptable values include None and "1" through "9".
So it appears that there are only 9 possible shortcuts. But there are more than 9 styles I commonly use. Is it possible to define additional such shortcuts?