How to force ifup to wait for dhcp?

[Mike Bird <mgb-debian@yosemite.net>]

How does one force ifup to wait for dhcp?

The Lenny laptops here start wlan0.  AP authentication
and DHCP both occur independently in the background.
Up to half a minute elapses before AP authentication
has occured, DHCP has retried, and DHCP has succeeded.

By then, openvpn has gotten very confused by the absence
of a default route and networking/openvpn/quagga have to
be restarted manually in order to put the pieces back
together in the right order.

Do I really need a "post-up sleep 30" or is there a better
solution?

TIA,

--Mike Bird