Dlink makes one, but not with the removeable antenna. You might consider going with somthing like a linksys WET11 wireless to ethernet converter (a little large, but has a screw mount antenna) or a linksys w54g-L (kinda bigger thena the WET--access point, but runs linux and can be made to do nifty stuff)
The wet 11 seems a little hackable (can apparently be made to run off AA batts, and can certyainly take a bigger antenna, etc)
Linksys also makes a 54g wet, but it looks pricey.
If you are talking about a bigger antenna, then maybe you aren't talking about pocketable stuff, right? I'm thinking about connecting my wet11 to my z for mobility with range (in the car) myself.