DVD-RAM is intended only for temporary storage. The phase change alloy that makes up the disc will break down in time -- sometimes in a matter of months, used or not.
On the other hand, dye-based media is intended for much longer shelf life, which includes DVD-R and DVD+R. A well-handled store-indoors DVD will last at least 25-50 years. Vetted tests by respected archival researchers have concluded at least 90% of discs will be readable in 40 years, if not longer. Those are all conservative numbers, too. You could have a higher % of discs stay good for much longer.
So "future" really depends on how long the future is. For something you plan to edit in a few days, use a DVD-RAM if you want. For something you may keep for a few years before coming back to it, then definitely go with a DVD-R.
Of course, you may want to abandon DVD-RAM altogether. As you have seen, it's not the most compatible format, being in the DVD-VR spec instead of the DVD-Video spec.
The videos are compressed in MPEG-2 format. iMovie doesn't natively support this. I've been (sadly) Mac-less for almost a whole year now, so I can't run any tests. (Know anybody that may want to donate, trade or sell an Mac mini, Powerbook or other hard cheap!?)
The Quicktime Pro codec may allow MPEG-2.
Per Apple, Quicktime Pro DOES NOT allow iMovie to import an MPEG2 video file. more on that from http://support.apple.com/kb/HT2657
It appears iMovie '08 may have some limited MPEG-2 support, so which version are you using? iMovie '9 may or may not allow MPEG-2. Because Apple is known to downgrade features in software/OS sometimes, it's hard to say without testing it out.
Apple purposely limits iMovie/iDVD -- they want you to spend $$$ on Final Cut Express
, Final Cut Pro or Final Cut Studio.
Do let me know what you find out!