just about to play Metal Slug 2 with my cousing online (multyplayer) with Kaillera.
Xmame dropped kaillera long ago,so now I'm running
http://www.kaillera.com/files/mame32-067-414-ka09.exe with Wine....
and its kaillera client works without a problem here on linux. I only had to set it up on windowed mode...
Oh and btw, It even picks up my joypad

and with sound,and almost no lag

by God, Wine devs are good.
Wine also works on another windows emulator with kaillera's netplay capabilities:
JneS. So if you wanna play some old school NeS game with a friend somewhere on the other side of the world, get jnes and open it with wine (again,it picked up my usb joypad!!)