I confirm this bug is in 10.6.8 too.
set n to "4.1" as real
set n to -n
produces the proper result

On Fri, 12 Apr 2013 12:30:08 +0200, Andreas Kiel <[log in to unmask]> wrote:
>
>Hi Mark,
>
>This annoying error has been around since a long time.
>I always converted "text numbers" to "real numbers" and did check the decimal divider used with the current system.
>You can figure out that with a short test
>set testDiv to 1/2
>The result may be 0.5 or 0,5
>
>- Andreas
>
>On Apr 8, 2013, at 8:05 PM, Mark Lively wrote:
>
>> 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