You can't sync or drag your non iTMS music back directly from an iPod to iTunes. The transfer of non iTMS content is designed by default to be one way from iTunes to iPod, the transfer of purchased content has recently been introduced with iTunes 7. However there is a manual method of accessing the iPod's hard drive and copying songs back to iTunes on Windows posted in this thread:
MacMuse - iPod to iTunes
If you prefer something more automated then there are a number of third party utilities that you can use to retrieve the music files and playlists from your iPod, this is just a selection. Have a look at the web pages and documentation for these, they are generally quite straightforward. As iTunes 7 is just out some of these may not have caught up with it yet though I haven't experienced any problems with the Mac software I use.
YamiPod Mac and Windows Versions
iGadget Windows Only
iPodCopy Mac and Windows Versions
iPod Access Mac and Windows Versions
PodUtil Mac and Windows Versions
iPodCopy Mac and Windows Versions
PodPlayer Windows Only
CopyPod Windows Only
For more on the new function in iTunes 7 look under under File>"Transfer Purchases from iPod" which transfers iTMS purchases. A paragraph on it has been added to this article:
How to use your iPod to move your music to a new computer