I noticed the same softness with the 24-105 that I just bought and ultimately I decided to return it and only use prime lens for now until a new zoom update comes out maybe like the 24-70 2.8 II (hopefully with IS)!
I shoot Weddings with my 24-105 on my 5D mark 2 and find the lens focusing very inconsistent. Images taken in a sequene with the same subject, composition and focusing point, some are sharp and some arn't. Generally by saying sharp I mean usable, I wouldn't say this is a sharp lens. For a while I just thought I hadn't got a great copy, bu :(t when a fiend of mine also got a 5D and 24-105 he has had the exact same problems, both of our lens's seam to experiance more softness at the long end. Was thinking of trading up for a 24-70 with the hope it would be better? Is there anymore of you guys with a similar experiance with this lens? as it has been for me unfortunately a bit disapointing.