I think some spammers have already caught up on this one.
Anyone can do a SHOW MX on an address and get one or more
MX records. Normally, the highest-priority should be taken,
and if that fails, move on to the next etc. Apparently, some
spammers are selecting them randomly, or even going for the
lowest-priority one on the assumption that the most spam checking
will be done on the highest-priority records. (This assumption is
not always true, of course, but as long as it is true more often than
it is false, it is worth it to the spammers.)

The large MX network that you mention, though, can be used by me
only if I use it for my MX records. It won't help me at all if
I continue to receive email directly (i.e. not only receive it
directly on my computers, but have someone sending me email make
a direct connection to my port 25).

If this network comes into existence and I choose to use it, then
in effect I am letting someone else decide what gets through and
what does not. Apparently SOME spam would still get through.
If I am going to let someone else decide anyway, I don't see any
reason not to choose someone who will just reject spam outright
(rather than throttling it), according to criteria I choose (my
dynamic-DNS provider offers such a service). I don't have any
more qualms about dropping connections which are a) from known
spamming IP addresses or b) which are trying to transmit spam
outright than you do setting up spam filtering for comments to
your blog. You don't let spammers and bots into your blog because
it is "free speech"; why the different stance on email? Yes, your
scheme would allow someone to send legitimate mail from any IP
address. How often does one need to do this? I think most people
send from the same computer most of the time (if it has a dynamic
IP address, then this "same computer" might be one other than their
own) and/or use SMTP-authentication.