The real question I have is not about Powell specifically, but whether or not it's physically possible to run that fast without drugs. There are many levels of doubt one can raise, but let's take them individually.
From an analytical point of view, we know the track, the wind, the elevation and the weather can also play a large role in the results of a sprint. The previously-mentioned Prof. Jonas Murieka's online time corrector is now able to factor in temperature and relative humidity as well as wind speed and altitude...yikes. He's got an all-time "adjusted" world list, but I don't think he's go the temp & humidity in there yet. I'll use this one to debate the merits of the athlete rather than the time. Here it is, with discussion on each athlete.
1. 9.80, Maurice Greene (adjusted from 9.79)
Bo Schembechler used to say he cold tell if someone was using steroids just by looking at him. Greene does have the ridiculously overdeveloped shoulder girdle that draws suspicion. However, it's his coach John Smith which truly makes me believe Greene's performances were not legitimate. Charlie Francis, Ben Johnson's coach, has been quoted as saying Smith "taught me all I know about steroids", and Smith was also involved in the Tim Montgomery / BALCO "Project World Record". The fact that none of his athletes have ever tested positive doesn't necessarily mean his charges are clean; more likely it indicates Smith is as wise about avoiding detection as anyone.
2. 9.84, Ato Boldon (adjusted from 9.86)
Also trained with Smith his whole professional career.
3. 9.85, Ben Johnson (adjusted from 9.79)
Serving a lifetime ban.
3. 9.85, Bruny Surin (adjusted from 9.84)
I've never heard anything on him one way or another.
3. 9.85, Asafa Powell (adjusted from 9.77)
The subject of the current speculation.
6. 9.86, Justin Gatlin (adjusted from 9.77)
The subject of much recent attention.
6. 9.86, Tim Montgomery (adjusted from 9.85)
Admitted drug use to the BALCO grand jury
6. 9.86, Frankie Fredericks (adjusted from 9.86)
Never heard anything about him; my gut feeling is "clean as a whistle"
9. 9.89, Donovan Bailey (adjusted from 9.84)
This was run on a ridiculously hard track which probably never should have met IAAF record standards. Bailey never ran remotely close to this in any other race. Should not be on an all-time list.
9. 9.89, Shawn Crawford (adjusted from 9.89)
More overmuscled than Greene, and trained with Trevor Graham. Almost assuredly a doper.
9. 9.89, Leroy Burrell (adjusted from 9.97)
Trained with Tom Tellez's Santa Monica Track Club; no allegations have ever been made against this group. However, Burrell was fitted with braces as an adult, and a re-adjusting jawline is one side effect of HGH use.
9. 9.89, Calvin Smith (adjusted from 10.04)
No allegations have ever been made against Smith as far as I'm aware. Like Fredericks, his small size leads me to believe he was a clean athlete.
That's all the sub-9.90-equivalent athletes ever. Is it possible to run that fast? Yes, but it's difficult. You need the right conditions and the right track. It most certainly should not be common.