Two tables refenceing each other's columns

Is something like the following allowed (or is not a Bad Idea)?

table1
---------+-------------------------
id1      |serial primary key
col2     |int references table2(id2)

table2
---------+-------------------------
id2      |serial primary key
col2     |int references table1(id1)


Obviously, creating the tables is a problem since the constraints require
that the other table exists.
If doing the above is *not* a Bad Idea, how could I work around this
problem?
(That is, (how) can I add the constraints after table creation?
I imagine something with "create constraint trigger", but the manual is
not very clear on that.)


Thanks

dan