The PIX won't route traffic back out the same interface it came in on, this includes IPSec traffic coming in from one tunnel and being routed back out another. This will work with a router as your hub, but not a PIX.
Note that in the upcoming v7 release (currently in beta), this restriction will be lifted and you'll be able to use the PIX as a VPN hub.
For spokes with dynamic addresses just define a dynamic crypto map as usual. The spoke's crypto ACL should include all traffic to the hub and to other spokes, this way the hub PIX will create the inverse of this for it's own crypto ACL and the traffic from spoke to spoke should be sent correctly (only in v7 though).