Andrew,

There are lots of reasons why she could be uninterested

Are her hormones still out of whack? (Have her see a doctor)
Did she have a difficult delivery and sex is uncomfortable? (Again see a doctor)
Is she self conscious because she hasn't lost all the baby weight? (If so, start complimenting her like there is no tomorrow)
Or is she exhausted from having a new baby and just overwhelmed? If so, help her out..

With women intimacy starts in our heads:

Talk to her about anything and nothing.
If the baby isn't sleeping through the night, alternate nights with her getting up with the baby so she can get some sleep.
Run a few loads of laundry.
Cook a meal
Cuddle on the couch
Take her out on a date.

A lot of times mothers of newborns get stuck in "mom mode" and forget about "wife/fiance/girlfriend mode".