The ifup-aliases script does not check if the gateway (in /etc/sysconfig/network) is on an aliased interface. While not common, it turns out to be a major inconvience when the primary interface is the internal network (ie. 10.x.x.x on eth0) and the aliased interface (DSL, cable modem, etc) has the gateway on it. The "solution" would be to put the gateway interface on eth0 and have eth0:0 be the internal network, but this could conceivably fail if the gateway interface fails to come up and then the aliased interface can't be created.
This issue has been forwarded to a developer for further action.
You should be able to change the 'GATEWAYDEV' parameter in /etc/sysconfig/network. ifup-aliases has been rewritten for the next release, and it should fix this.