Re: Spam, Spam and more spam
The short answer is Yes. The long one is they are made for filling out forms and submitting them. A drop-down is just a field that they might encounter, so expect the functionality. On the other hand we are talking here about bots that look for WP/MU installations to exploit the default sign up or comment forms. As a rule of thumb, anything that you can do to change the default behavior, do it. It’s like Andy said: if you make it the default, the spammers will figure out a way to get around it.
Also: try very hard to stay away from the following in your URLs: wp-signup.php, wp-register, register, wpmu, wp, and anything that hints at a wordpress installation.