Negative numbers for PK/ID's?

Hi,

A while ago on a different SQL platform, I had the idea to use negative 
numbers as id's for certain system records that I didn't prefer to have 
interspersed with other records in a table. (For example, we had 
"template" records which we used to spawn new records, and rather than 
store them in a different table we just had records id=-1,-2 etc and 
did an insert/select to create new "live" rows based on the templates). 
Preventing them from displaying in the results was as simple as 
ensuring that all select statements had a "id > 0" statement in them..

I'm wondering if there are any Bad Things that happen if I use negative 
integers for primary key values in Postgres (v8.2)? My primary keys are 
all bigserial type.

Any comments or advice?

Thanks!

Steve