Let-bindings in SQL statements

Hi,

Is it possible to do the equivalent of let-bindings in a pure SQL function?
I have a SELECT that invokes "now" multiple times.  It would be nicer
to do it only once and reuse the value.  Something like this:

LET right_now = SELECT now () IN
SELECT * FROM my_table WHERE right_now >= start AND ...

In PL/pgSQL this is easy, but I wonder about SQL...

Thanks in advance!
Jon