looking around the internet i've seen two reasons why some chargers won't change the iphone 3G
1. The pins that charge the iphone on the dock connector have changed, apple have dropped the firewire ipod cable, so some pins have changed in operation
2. apple software can determine (I'm not exactly sure how) if accessories are genuine "Made for iPod" accessories or not, if not then they won't work
So cheap accessories from China aren't likely to work due to these reasons, most accessories from reputable companies are "Made for iPod" however, and 'should' work, providing they are suitable for iPhone 3G (due to iPod Dock connector pin changes)!
Apparently Apple did tell manufactures that the firewire cable was being discontinued, and would no longer be supported, but some manufactures still made products that worked the 'old way' and not the 'new way', thats why some old chargers (and new) wont charge some new apple products, e.g. iPhone 3G
I'm still after a suitable cradle if anyone can recommend one?