By downloading the SD versions of the WWDC videos, you can have a complete set to watch much sooner than if you download the HD versions. Also, you have a much better chance of fitting the entire collection on your iDevice if you use the SD versions.
Here are the 2012 videos:
(This year Apple took only four days to make the videos available to all registered developers, which is really amazing. Thank you, Ron Okamoto!)
If you follow the link above you get to the HD versions of the videos in iTunes. That's a huge amount to download. If you click the "Developer on iTunes" breadcrumb on the upper right, you can get to the SD versions.
How much less are the SD videos to download than the HD ones? Doing a Get Info in Finder, I see that in 2010 the HD videos were about 32GB total, compared to 14GB for SD. Big difference, especially if your Internet access is not super-fast.
In 2011 Apple must have increased the resolution of the HD videos, because they totaled about 53GB. I assume the SD was again roughly half that, which would mean an even bigger size difference than in 2010.
iTunes says there are only 210 SD tracks for WWDC 2012, compared to 224 HD ones. ("Tracks" seems to include the PDFs of the slides, so really it's about 110 videos.) I don't know what's missing on the SD side.
In any case, I plan to download both sets, starting with SD so I'll have a complete set that much sooner. I do want the HD versions because they are noticeably nicer to watch. I seem to recall code samples sometimes being hard to read in the SD versions.