My friends and I did Alice in wonderland for our school play and wanted to figure out the answer. After about a month of thinking we figured it out. A raven is like a writing desk because Edgar Allen Poe wrote The Raven on a witingdesk.
All the best, madwriter (Natassia Gaines) Lost Creek, WV
ETA: Link to column: http://www.straightdope.com/columns/read/1173/why-is-a-raven-like-a-writing-desk