In the past you had to use ndd to do that.

Thanks solaris 10, the new dladm command do the trick.

dladm show-dev
bge0            link: up        speed: 1000  Mbps       duplex: full
bge1            link: up        speed: 1000  Mbps       duplex: full
bge2            link: unknown   speed: 0     Mbps       duplex: unknown
bge3            link: unknown   speed: 0     Mbps       duplex: unknown

It also gives you some usefull vlan tagging infos:

dladm show-link
bge0            type: non-vlan  mtu: 1500       device: bge0
bge1            type: non-vlan  mtu: 1500       device: bge1
bge2            type: non-vlan  mtu: 1500       device: bge2
bge3            type: non-vlan  mtu: 1500       device: bge3
bge244001       type: vlan 244  mtu: 1500       device: bge1