I keep my contacts in LinkedIn. I consider it my master source, and I try and have all other Web 2.0 apps import from there. Twitter does not have an import-from-LinkedIn feature.
I happened to be trying to look up a LinkedIn contact's email information, and when I saw the link to download a vcard, I wondered "hmmm... I wonder if I can mass download all my contacts?" So I did a little digging, and sure enough you can export your contacts from LinkedIn.
In doing so, LinkedIn gives you a number of export options, including Outlook, Yahoo, Mac OS X address book, and vcard. Twitter can scan your contacts from GMail, Yahoo, AOL, MSN, and Hotmail. The common link is Yahoo, so you will need to have a Yahoo account to pull this off.
I just exported from LinkedIn, imported into Yahoo (LinkedIn gives directions on how to do this), then went to Twitter and imported from Yahoo. Worked like a charm.