Since coreutils 8.1 (in Squeeze, not Lenny), there is a command that simply prints out how many processors (cores, processing units) are available:

$ nproc

The use case is obvious:

$ make -j $(nproc)

On a side note, this is gnulib's nproc module wrapped into a C program. If you didn't know gnulib before (it had slipped my attention until recently), it is a library of portability functions and other useful things. Adding it to a project is simply done by calling gnulib-tool and tweaking a few lines in the automake/whatever build scripts.

PS: do not use nproc unconditionally in debian/rules. Parse DEB_BUILD_OPTIONS for "parallel" instead.