The difference is simply the driver used to handle the NIC. Both bge# and e1000g# would map to eth# on linux.
Assigning a virtual IP can be done with the ifconfig command, eg:
ifconfig bge0:1 plumb
ifconfig bge0:1 192.168.1.2 up
ifconfig bge0 addif 192.168.1.2
For persistent assignations, put the IP address in the /etc/hostname.bge0:1 file.
Note that Solaris 11 no more distinguish interfaces names by default and uses net0, net1 and so on. Solaris 11 also introduces a totally different command set to administrate the network interfaces.