Usually, when Premiere Elements Media Downloader is used to capture the video and you have the latest version of Quicktime, this is not the case. The clips do show up.
But this is not always the case on all systems.
Have you contacted Adobe to find out why this might be happening? This is just a user-to-user forum. All we can speak to is our own personal experience.
I have contacted Adobe support but they did not have an answer. I am running the latest Quicktime.
I am having the same problem on both systems that I run Premiere Elements 8 Organizer. All software and drivers are up to date.The only way that I can get thumbnails to display for AVCHD files in the Organizer, is to change the file extension from .mts to .m2ts. In my opinion, that is clearly a bug with the Organizer that must be fixed by Adobe ASAP. Organizing video without thumbnails is next to useless.
If you start a new project and open Premier Elements you can create the thumbnails for AVCHD files with the .mts extension by right clicking each one and selecting Update Thumbnail (which is laborious, at best). However, when you exit the project and go back to Organizer the thumbnails are still not there, nor can you create them. Next, go back into Elements and the thumbnails are there.
Can you say bug!
Update 07/15/10: I contacted technical support and they said that this is a known problem. The only solution, if you could call it that, is to use file renaming utility to change all your file extensions from .mts to .m2ts. I hope they fix this problem and release an update soon, as all I shoot these days is AVCHD with the Panasonic HDC-TM700K.