From my experience, post-interview offers are usually a phone call. Everyone I know, myself included, have only received rejections in the mail.
Wait, I should have clarified this before hand, but are you guys talking about summer clerks or full time attorney positions?
Summer jobs are of definite duration (i.e. you are being hired for a SET TIME ONLY), phone calls (and more often e-mails) are the norm.
Full time attorney positions (ie. of indefinite duration should ALWAYS be in writing, if they are not be very leery of accepting them for the obvious reasons that will bore you to death if you take Employment law).
Since I have graduated I assumed you were talking permanent positions. That's what I get for imposing my own prospective without clarifying first. But for permanent positions you will always get a written offer snail-mail style (you might also get a phone call or e-mail before) but firms and lawyers like paper trails. If you don't get a firm written offer then be leery of accepting or committing to anything until you do. Every offer for (post graduation) employment I have had has either been written offers or verbal offers followed by written confirmation of the details of the offer.