In 6.0 this was not a problem. In 6.1 using the Xircom Combo cards, the modem will not function while the Ethernet part is working. As soon as you ifdown eth0 the modem will start dialing.
In your /var/log/messages file, are you getting warnings about your default route? This is likely pppd refusing to replace the default route if one already exists.
It can't activate the port. Kernel updates seem to fix the problem.
This is a change in pppd behaviour. The next release of initscripts will work around this change.
Whether this particular problem is a pcmcia problem or the problem I thought it was in initscripts, it is fixed now.