I don’t mean to spread false rumors. but this is my personal experience.
It’s not false, it’s a fact, and it’s stupid. There are ways around it. You could, as bupla suggests, duplicate Blender a bunch of times. You can run six “instances” of Blender if you’ve got Blender installed six different places on your machine. Which is one reason why on Mac I usually keep several different versions at least accessible, even if I only really want to use one. However, clicking on a .blend will still try to open the default installation, which cannot be opened if another .blend is already open.
You can also write a script that runs Blender from the command line, where it’s possible to run multiple instances (that means, don’t blame FreeBSD for this dumb limitation… it’s entirely OS X’s fault). However, although I managed to get a script that could execute by double clicking on my desktop, I was not able to figure out how to make this work with double clicking on a .blend file, which defeats the whole point.
Basically, there’s no way I know of to set up a Mac so that a .blend file always opens when you click or download it. If you learn from .blend files, or if you often try to help people by opening up their .blend files while doing your own work, this is a pain in the neck. I almost always have multiple instances going, so Mac really cramps my style.
Kind of like After Effects.
Actually, it’s a Mac thing. OS X won’t open up multiple instances of any program, but most programs (like Firefox, MS Word, etc) can open up multiple documents within themselves. There’s been some talk about making Blender able to handle multiple docs, and this might, might come as a result of the changes in 2.5. Although I don’t think it’s considered to be a priority.