It seems that there are still people having difficulties getting their dual layer dvds to play. I suspect that the majority of these are related to the disc burning process and ancillary equipment rather than the Wii itself.
I initially used a DVD -R DL from Verbatim rated at x4 which I burnt at x2 speed and seemingly burnt flawlessly with no layer break and was verified OK by IMGBURN however I got the #001 error message when I tried it after it has successfully upgraded my firmware. Interesting my Wii was previously at 3.0U (with wiikey 1.9g) and only upgraded it to 3.1U.
I subsequently upgraded to 3.2U wirelessly before doing another disc.
My second dics was an Imation DVD +R DL rated at x2.4 which I burnt, with no layer break, and verified at x2.4. This disc has played flawlessly so far.
A few things worth noting for people having difficulties:
1. If you have a NTSC Wii then you dont need to use the layer break
2. Make sure your Wii firmware is at either 3.1 or 3.2.
3. Make sure your burner firmware is up to date and using the correct drivers
4. Use quality +R DL media - check this website to be sure you do have quality media (dont just rely on brand name but actually check the media ID as well)
http://www.digitalfaq.com/media/dvdmedia.htm
5. Very importantly, make sure your are in DMA mode not PIO for your burner when in use (check under IDE controllers in control panel if you have an IDE burner)
6. Under no circumstances have any other activity going on in your computer when you are burning.
If the burn is not successful and you get an error message on your Wii you could always use something like CDspeed2000/Nero CD DVD speed to scan the disc and try and find if there is a problem with the disc burn.