Randomizing the field names is a great idea but as you said they would just need to scrape the HTML each time they wanted to register. Have you considered sprinkling in random bits of markup to throw off the people using regex and other lazy parsing methods? That might make it a real pain to scrape your forms depending on how the spammer parses your page.
I still have 'username', 'email', and 'password' fields in the form but I hide those elements with CSS, which no scraper is going to bother parsing. When the registration form is submitted the account is essentially hellbanned, they can 'activate' the account via the normal email confirmation process but anything they post disappears into the ether.
I'm catching about 100 spam accounts a day with this technique[1] and the ones that I miss are fairly easy to detect through analysis once they start using their account.
What happens if someone uses something like LastPass, RoboForm, or any of the other automatic form fillers to legitimately sign up for your website? I would imagine that these would "guess" that username means username and email means email, which may lead to false positives for real users.
One way to avoid this issue might be to plop a hidden input box on the page if this input has text in it when it's submitted, you silently drop the registration.
