We have found some serious issues with precision regarding rigged meshes. For example rigged attachments may get attached slightly away from their expected location on the character.
This issue seems to appear only if you create attachments for your SL Avatar. As long as you work with full characters and as long as the charater attachments use the same armature as the main character, these precision issues are either neglectable or do not show up at all.
Our current experiments point to some scaling issues with the Blender Collada exporter. We still need to find the cause of the problems. We will release a fix as soon as we got it. Sorry, there is no time estimate available for when the problem will be solved. Please be prepared for a few weeks.
Note: The roll out of the blender 2.63 support is not affected. So you will get Avastar for Blender 2.63 within the next few days.