I read this on the Web regarding the Sigma 18-50 f/2.8 and Canon Speedlites:

"...the AF does not work right when using an external flash (such as the 550EX or 580EX) and the AF Assist Lamp on a 20D (don't know about other bodies)."

Does anybody know if this is true?

Why would you use the AF assist lamp on the body if you have the external flash?
The 580EX uses a red line pattern projected as a focussing target which would compete with the camera's AF assist lamp.
I think that the EX flashes and the 20D work together so that only one provides the AF illumination.

I've not encountered it myself, but I've seen reports some Sigma lenses have chips in them that don't work with newer Canon bodies, and they are upgradeable by Sigma (but I have read it can be expensive).

I think we'd need more specific information about what exactly the problem is. This doesn't quite add up for me. The lens and flash don't communicate directly, although the camera communicates with both. If there's an auto focus problem while using a flash, my guess is that there's an auto focus problem - period. Unless there's some sort of electronic conflict between the 580 and that lens - which seems very unlikely - the problem is probably with the lens programming/chip.

Have you checked the reviews to see if anyone mentions this problem?