Create interval using column value?

Hi

I have a table (A) with a integer column called build_interval, which 
contains the number of milliseconds betweeen builds.  I have another table 
(B) with a timestamp with timezone column called built_on.  The two tables 
share a column called join_col.  What I want to do is find all the rows in 
A which were built before now() - build_interval seconds ago.  If I could 
just write this in SQL it would look something like:

SELECT A.id FROM A JOIN B ON (join_col) WHERE built_on < now() - interval 
'build_interval seconds';  This does not work at all - I cannot find a way 
to build an interval using a column - casting an integer as an interval is 
forbidden. Can anyone suggest a way to achieve my desired result? I'm not 
wedded to the interval approach.  I can probably change the column type of 
build_interval if necessary.

I am using postgres 7.3.4

Thanks in advance for any help.