Can't figure out how to use now() in default for tsrange column (PG 9.2)

I'm trying to set [now(), 2049-12-31 00:00:00) as the default for a
tsrange column (Postgres 9.2), but can't figure out how to do it. I'm
either getting syntax errors or now() is being evaluated, so that the
default becomes something like [2012-07-14 14:04:35, 2049-12-31
00:00:00), which is not what I want. Can anyone point me in the right
direction, please?