OK.
Some serious reasons not to boycott the Olympics.

1) Punishing China for the actions of Burma and Sudan is unfair. Like a teacher punishing the whole class for the actions of a few.

2) A boycott would be divisive. If a seious boycott campaign began, we'd spend all our time arguing about whether to boycott and the human rights issues would be sidelined.

3) The West has very little moral authority now. Invasions of Afganhistan and Iraq, little restraint on Israel for bombing Lebanon etc. A boycott could be seen as another example of Western hypocrisy - and could push some countries closer to China/Russia etc.

4) Deadline politics is bad politics. The boycott campaign effectively wants China to solve Burma/Darfur/Tibet/Taiwan/Falun Gong etc all by 8/8/08.
This is insanely optimistic. Also linking so many campaigns to the Olympics means there is serious risk of the campaigns blowing themselves out once the Games are over.
We need a more considered longer-term approach.

5) Do we really want to antagonise China? - snubbing or humiliating China would not be a good thing for international realations. A boycott would in my view create an angry and resentful China which would not be a good thing for the world.
There are numerous ways to stand up to China but deliberate humiliation is not the way to conduct international politics.

6) This could be seen as a snub to the Chinese people. I'm sure some will say that millions of Chinese are against the Games. They are probably related to the thousands of Iraqis who were going to cheer the Americans going into Baghdad.
I'm sure most Chinese want their Games to be the best ever. Snubbing them could make them rally behind their government and support a more beligerent post-Olympic China.

7) Human rights in China are a problem as stigweard points out but they were also a problem when the Women's Soccer World Cup was held there, also the Formula 1 Grand Prix, the Beijing tennis etc.
Small, regular events like these do more to legitimise China than a single event like the Olympics. This proves that the boycott is merely opportunist.

8) The Olympics will be propaganda for China. Nonsense.
Agreed, the Games put China in the spotlight, so counter propaganda is being used everyday against China. By the time Beijing 2008 comes around, the world will know what bastards the Chinese government are. Why spoil it by having a boycott.

9) I still believe the Olympics can be a force for good. Olympic solidarity gives money to poor countries to develop sport to attempt to level the playing field. A successful Olympics may help in the opening up of China. Especially after China's pre-Olympic paranoia has gone. I say may because nothing is certain.

10) Finally, the athletes,. It does seem very unfair that Soccer players, tennis players, motor racing drivers are able to pursue their lucrative jobs unaffected by boycotts, buit the little guys like archers, rowers, judo players etc for whom the Olympics is their only taste of the big time lose out.
Boycotts hurt the little guys in sport and leave the rich unaffected.