Actually, I'll be the devils advocate here, because I actually enjoy the challenge of this stuff from both sides, and because a lot of you jokers are just lazy.
I worked with a guy once who refused (categorically) to upgrade to any browser past IE 6, and he held to that policy through hell, high water, and tears, until our own website would no longer even work on the browser installed on our internal desktops. He had the exact same view about everything else on the network that he didn't put there himself, and was just a complete misery to work with.
As an admin, your job should be to help the users do their job, and to try and help out those few who have some technical competence when it's not going to be too much trouble. This is not to say that you should let every schmuck come in and plug in whatever the hell they want, and it's not to say that you should compromise your security in the name of user convenience, but your job is not to keep your network wholly pristine, it is to make it as useful as possible, and useful means getting a little dirty.
The problem is always going to be the extra IP address/Mac Address pair. That's what they're going to be looking for: even if they don't have DHCP completely locked down, they're bound to be scanning for "new" MAC addresses. You can't secure your physical network if you don't watch out for unfamiliar MACs.
To get around it, either you have to route the extra traffic through your machine, using some kind of proxy (which is only possible if you can install software), or you're going to have to slip a switch in between your machine and the network and then NAT all the traffic straight through to your desktop (chances are they have RPD or similar set up so they can admin your machine remotely), and you're going to have to configure the switch to have your machines IP address, and your MAC address, and you may not be able to determine these depending on how locked down your machine is.
Now, both of these solutions are going to be completely obvious to anyone who bothers to leave the coffee pot and walk around. If you're lucky, the guy is going to think the rule prohibiting you from connecting your virus-laden tablet PC to the company network is outdated and turn a blind eye. If you're unlucky, they'll disconnect it with a hammer (I have actually done this to an open WAP that someone plugged into my network...Watching them try to complain about it was delicious).
Your best policy is to go to your boss and sell them on your peripheral. Then have them go to the guy who is over both your department and IT, and sell them, and then have them dump it Wrath-of-God style on the IT people, who will then cite chapter and verse on why allowing your device onto the network will end civilization as we know it. Depending on which boss sells it better, it will/won't get done, and, either way, eventually, the admins may take you off of their shit list.
What I would do, wise as I am in the ways of networks, is ask for an external WAP. If you pitch one that is outside the internal, protected network, you can probably sell it because of all the execs who have Crackberries and iPhones, who will appreciate the wifi. And it's the sort of thing that doesn't cost very much, and soothes a lot of employees, that IT bosses like to do.