Very interesting topic.

re: Apophis

According to NASA the "Impact Probability (cumulative)" is 2.2e-05
http://neo.jpl.nasa.gov/risk/a99942.html

which means:

0.002200000% chance of Earth impact,
or, 1 in 45,000 chance
or, 99.99780000% chance the asteroid will miss the Earth
http://neo.jpl.nasa.gov/cgi-bin/ip?2.2e-05
_________

I've found these worth reading:

Workshop on Scientific Requirements for Mitigation of Hazardous Comets and Asteroids [2002]
http://www.noao.edu/meetings/mitigation/report.html
- click the site link for PDF or HTML

NEO Impact Symposium & Secrecy Issues [2003]
http://impact.arc.nasa.gov/news_detail.cfm?ID=122

Optimal Trajectories from the Earth-Moon L1 and L3 Points to Deflect Hazardous Asteroids and Comets [2004]
http://www.annalsnyas.org/cgi/content/abstract/1017/1/370

The Torino Impact hazard Scale [wording revised following publications up to 2004]
http://neo.jpl.nasa.gov/torino_scale.html


"Time is what prevents everything from happening at once" - John Wheeler