Edit: do I (and others like me) have to keep a very close eye on build times, unreadable code and lack of portability and do massive prototyping to ensure that it's safe to move on with the new standard?
Yes. But you have to do all these things with the current standard as well. I don't see that it is getting any worse with C++0x.
C++ build times have always sucked. There's no reason why C++0x should be slower than it is today, though. As always, you only include the headers you need. And each header has not grown noticeably bigger, as far as I can tell.
Of course Concepts was one of the big unknowns here, and it was feared that they would slow down compile-times dramatically. Which was one of the many reasons why they were cut.
C++ easily becomes unreadable if you're not careful. Again, nothing new there. And again, C++0x offers a lot of tools to help minimize this problem. Lambdas aren't quite as concise as in, say, Python or SML, but they're a hell of a lot more readable than the functors we're having to define today.
As for portability, C++ is a minefield already. There are no guarantees given for integer type sizes, nor for string encodings. In both cases, C++0x offers the tools to fix this (with Unicode-specific char types, and integers of a guaranteed fixed size)
The upcoming standard nails down a number of issues that currently hinder portability.
So overall, yes, the issues you mention are real. They exist today, and they will exist in C++0x. But as far as I can see, C++0x lessens the impact of these problems. It won't make them worse.
You're right, it'll take a while for compliant standards to become available on all platforms. But I think it'll be a quicker process than it was with C++98.
All the major compiler vendors seem very keen on C++0x support, which wasn't really the case last time around. (probably because back then, it was mostly a matter of adjusting and fixing the pre-standard features they already implemented, so it was easier to claim that your pre-standard compiler was "sort of almost nearly C++98-compliant".
I think on the whole, the C++ community is much more standard-focused and forward-looking than a decade ago. If you want to sell your compiler, you're going to have to take C++0x seriously.
But there's definitely going to be a period of several years from the standard is released until fully (or mostly) compliant compilers are available.