It works, no problem. The PIX crypto config should point to the loopback address on the router.
The router crypto config will look like this:
crypto map mymap local-address loopback0
crypto map mymap 10 ipsec-isakmp
set peer
set transform-set ....
match address ....
The rest of the crypto config is standard.
The "local-address" command makes the router source all crypto packets from the loopback address, which is necessary because that's what the PIX is pointing to.
Put the crypto map on the OUTGOING router interface, NOT the loopback interface, and everything should be fine.