RTW Ticket Info
Most airlines price a Round The World ticket. Delta seems to have a fairly flexible pricing, about $2500 gets you 25,000 miles, with $100 per 1000 miles over. However, this restricts you to the northern hemisphere. Delta also prices a ticket with more miles for both hemispheres, but it is more expensive--in the $3400 range. While you must fix the route when you purchase, they earn kudos for allowing flexibility in the travel dates. Most airlines don't even offer that. My suggestion is to take the chance and buy your tickets piecemeal as you travel. This provides the most flexibility, and can even be the cheapest if you buy the tickets in the right place.
From the "Wonderful Wander" series in the LA Times:
Given our modest budget, the sensible source for airline tickets was one of the agencies specializing in around-the-world fares. The three leading U.S. companies in the field--Air Brokers International, High Adventure Travel and TicketPlanet--are based in San Francisco. Each can tack together a customized itinerary made up of deeply discounted one-way air fares on various well-known--and not-so-well-known--carriers. The resulting cost is often half of what you would pay by buying tickets directly from the airlines.
Air Brokers is the firm they ultimately chose. They have a cool trip designer which is worth checking out.