Well, first and foremost, any advice here take with a HUGE mountain of salt. Number 1 thing to do, which you really should do, is consult a lawyer. It'll save you a lot of grief and aggrivation and the lawyer will be the most knowledgeable in terms of the law.
Now I have heard of cases like these before, where ICANN has awarded domains to companies that own the trademark. However, in the US I believe, if I understand the law correctly, that multiple companies can hold the same trademark if they are in sufficiently different areas of buisness. For instance, there is a company around here called Apple Plumbing - now the software/hardware company Apple can't sue them for trademark infringement. Of course there could be other laws applying as well, I'm not sure. Common sense would tell me that if your two buisness are in separate markets, you should be able to keep your domain name - but you never know with laws.
So, yes, my advice to you is sit down with and talk to a lawyer. Might cost a bit, but it can cost even more if it gets messy.