1. Browse to My Music under My Documents and open the Zune folder.
2. Copy the zpl file you want to your Desktop and rename it to wpl.
3. Open the wpl with Windows Media Player (zpl and wpl are basically very similar)
All your songs from the playlist should be in Now Playing and no other songs.
4. Right-click on the title bar to get to the hidden menu or press CTRL+M for Classic Menus.
5. Go to File > Save Now Playing List As…, then change the format to m3u or wpl.
Source: Zunescene (archive.org)
You can also open m3u into Winamp. Then, using the gen_yar.dll (Yar-Matey! Playlist Copier v1.07) plugin you can export the files from the playlist to a specified folder, such as an SD card or flash drive.
Update: I found a program that will convert to m3u and pls directly from the file to a specified location.
Zune Playlist Converter by HazteK Software
Mirror: Download ZPL2M3U from Drale Blog