Since there has been some activity I want to share this warning.
In older version of Applescript (before 10.7)
set n to "4.1"
set n to -n
would yield -4.1
Under 10.7 and 10.8 it yields -4
It seems the default coercion is to integer rather than number.
bizarrely,
set n to -1 * n
yields -4.1
-Mark
Fixed my code, Documented my fixes, Filed a bug report.