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.