I noticed today the same problem on my Alum Macbook that I purchased less then a year ago.
I reported my problem to two different reps at Apple; tech support is aware of this "lid opening when at 90 degrees" problem and have it documented in their problem-based directories. Currently however, it is not merited as a warranty approved fix. Supposedly its been an issue only for Aluminum Macbooks manufactured in 2008. Newer Alum Macbook Pros seem to not be affected.
I've had Apple put a note on my file outlining my situation; the rep told me that if/when Apple decides to fix these issues, clients whose 1 year warranties have expired should still be eligible to have their Macbooks fixed.
I suspect they won't do much until enough people start complaining about the problem. For me personally, the problem is not so much that the computer opens when on its side but that when the lid opens it will come out of sleep mode and drain the battery.
The quick fix for those carrying Alum Macbooks in carriers is to pack them with the opening at the bottom. That is; with the hinge facing down so that the Apple logo is upside down when at a 90 degree angle. At first I thought this was a no no but was assured by Apple that the computers are designed to be packed either way.
Hope this helps. If enough people call in to Apple tech support they might do something about it.