Definitely make your volunteer application form robust and detailed -- this is your first crack at screening in volunteers who know how to make a commitment and know how to communicate via the written word, and screening out people who don't understand that volunteering online takes real time and commitment. If you make a skimpy expression of interest form for new volunteers, you can expect a lot of people expressing interest, but very few actually taking on and finishing assignments.
I think it's best to let volunteers self-select assignments -- let them see what's available and express interest in specific tasks, rather than your trying to match people to assignments. That takes a lot of management off you.
As for a drop-down menu, have a look at the various online databases out there to recruit volunteers and those to recruit paid staff, as they have so much in common in terms of applications. Look at sites like VolunteerMatch, Idealist, the UN's Online Volunteering service, DevelopmentEx, and ReliefWeb (vacancies section). Look at how assignments are posted, and how people complete online applications to both express interest and to show off their online communications abilities.
Also, read over the information at the Virtual Volunteering Project; although it's no longer updated, it still provides detailed information about working with volunteers online. The Virtual Volunteering Guidebook available for free on the site has been substantially updated and will be published, again for free, later in 2009.