If it takes off, there will be other versions of the hardware in future, similar devices from other manufacturers, and - I have no doubt - at least one attempt by the community to create their own open-source firmware for the original Pi. A pure open-source device may come at some point, but right here & now, just isn't feasible. Does that mean the Foundation should just give up, and not create this device at all? Personally, I don't think it does; there is still a hell of a lot that can be taught without getting into device driver development, and hence there is still a lot of value in shipping the device as it currently stands. If nothing else, it will prove to the world that it can be done, and go a long way towards filling the gaping hole left in UK (and worldwide) computer science teaching by the venerable BBC Micro.
Right now, the majority of children are growing up in a world where computers are seen as expensive, and as toys or appliances: they do what they were purchased for. Most children haven't heard of Linux or *BSD, and even if they have, installing it on a shared family computer is a risky prospect for someone who is still learning how these things work. There are free development tools for Windows, but again their existence isn't common knowledge amongst children, and IMHO they hide away so many of the low-level details behind GUIs and automatic code completion that they are in some ways harder to use (for absolute beginners) & less suited for teaching than their command-line equivalents. I believe there are also licensing concerns with the free versions, in terms of how you're allowed to distribute any resulting binaries.
The Pi gives children the chance to have their own computer, with officially supplied development tools and teaching materials, unencumbered by licensing concerns on the development tools themselves. For someone who learnt to program using the built-in BBC Basic on an Acorn Archimedes (a direct descendent of the BBC Micro), using the accompanying printed reference manual, and has watched in horror as computers morphed over the years into toys & appliances (where are the development tools on a fresh Windows install? Where are the reference manuals? Are Apple any better?), this is very much a Good Thing.
A couple of years ago, my younger brother asked me for advice on learning to program, and - because I knew that all he had access to was the family PC, running Windows - I didn't know how to answer. By the time my own daughter is old enough to ask that question, if she ever does, I hope to have a good answer for her - and not just because her daddy happens to know his stuff.