Celebrations based on sports themed weddings are an interesting concept and they do lend themselves well to a lot of decorating options. Just choose your favorite sport or team, then use the colors and symbols available to help build your wedding theme.
Invitations, decor, favors, cake design, colors, and often food choices, will all be centered around your favorite sport. If you both have a favorite team, then it should help make your planning even easier.
As an example, for a hockey themed wedding, you can either just work with general "hockey" decor - pucks, hockey sticks, skates, etc. - you can match the colors used with that of your favorite team.
For golf, green and white would be the best color choices and the decor will center around golf clubs, golf balls, tees, flags, etc. Your invitation could even resemble a score card.
For those who like the idea of having a marriage celebration based around a sport or hobby, here's a Going Fishing Theme Wedding that provides a variety of ideas on invitations, decorating and favors.
Any way you look at it, there's a huge variety of sports to pick from. Most couples already know what type of sports themed wedding they will be using, however for those of you who aren't quite sure what to pick, here is a listing of what is popular. (Note: I may have missed some, but this gives you a general idea).
Cross Country/Downhill Skiing
Dirt Bike Racing
Hot Air Balloons
Ping Pong/Table Tennis