fixit_f,
To authorise the connection of a new device/OS to the company's VPN the IT department has to audit the device to ensure it satisfies certain criteria. Going forward they would also be responsible for ensuring that future releases don't introduce any problems, as well as keeping up to date with patches and ensuring that all users do the same. This all incurs significant administrative overhead and possibly additional skills and expertise.
When they purchased the product from Cisco it came with certain assurances from the vendor that carry a lot of weight. A lot more weight, in terms of corporate risk management, then a bunch of academics on a mailing list extolling the virtues of this 'Linux' thing.
You may expect them to make an exception becuase you're 'special' and you 'know what you're doing', but it frankly isn't worth the risk (to the company, and the IT dept as individuals with responsibilies), and to do it properly isn't worth their time. If home working is a requirement then I suggest you install VMWare on your Linux system and then request the company purchase a copy of XP that you can install on it and use this to do your work.
(I use Linux and it's great and I wish I could install it on my work PC but I understand why I can't)