The global command IP addresses are what outbound traffic will be NAT'd to as they go out the PIX. I guess theoretically if this PIX is getting a dynamic IP address then you wouldn't expect it to use static IP addresses for NAT'ing, but it's also not incorrect.
Normally, on a dynamically addressed PIX you would use the following for your outbound NAT'ing:
nat (inside) 1 0.0.0.0 0.0.0.0
global (outside) 1 interface
The "interface" keyword tells the PIX to PAT all outbound traffic to whatever IP address is on the outside interface, which is dynamically obtained.
Keep in mind these two commands have nothing specifically to do with VPN access, they're only used for translating addresses from a higher-security interface (inside) to a lower-security (outside).
