First off, if you do not have a Blu-Ray player and just want to watch HD videos you created on you nice HD television you may just want to spend < $100 and get the Apple TV and avoid the hassle of buying a burner, blank disks, authoring, & burning. Just a thought.
How can I make a high definition (HD) DVD? Truth is there is no such thing. DVD's are standard definition (SD). They must be MPEG-2 video with a resolution of 720x480 (NTSC) or 720x576 (PAL). DVD players will only play actual DVD's. (More information on this) You can put HD videos to a file on a DVD but your DVD player won't play them*.
*The High-Def/Blu-ray Disc Plug-in for Toast 10 Titanium lets you author HD video content from AVCHD camcorders and EyeTV recordings on to standard DVDs and Blu-ray Discs for playback on any standard Blu-ray set top box. But, you will not be able to put very much footage on there.
Does iDVD support HD? NO, it makes DVDs. See above.
Does DVD Studio Pro make HD disks? NO.
What if I burn my HD iMovie project to iDVD? You will end up with a very good looking DVD but it won't be HD.
How can I burn HD and watch it with my Blu-Ray player? First off you will need a Blu-Ray burner.
Here is an internal one known to work with PowerMacs quite nicely.
Here are some external options DIGISTOR External Blu-ray Burner USB 2.0 or OWC Mercury Pro 12X Blu-Ray+ Bundled w/Toast Ti PRO
Then you are going to need either:
Toast 10 Titanium - $ (If it did not come with your drive)
Adobe Encore - $$$$
Final Cut Studio - $$$$
There are other devices that will play your High Def H.264 movie file including; Apple TV, Sony Playstation 3, if you have a DVD player like mine (or similar device) that lets you attach a USB drive you can watch them via that.
Note: Apple does not yet support Blu-ray movie playback on Mac OS X