This is a tremendously fascinating article analyzing (in a more-or-less armchair fashion) how many of Bonds's home runs may actually be chalked up to steroids.
The answer? 98, putting him at 616 currently. Still a prodigious number, but it would put him pretty far out from Aaron.
Anyway, the analysis is interesting, even if it may only be speculative, and the bits about baseball physics are pretty cool.