David Weinberger is up receiving more than 2000 pieces of spam every day. That's spam every 40 seconds around the clock. That is in short a lot of spam.
In term of download bandwidth, assuming a modest 5K a piece (it is almost certainly more) that's 10 MB of spam, which means Weinberger actually needs ADSL or better just to download his email. Using a modem it would take ~ 30 minutes daily connection time to just receive all the email.
Reading the comment thread on Weinbergers announcement is also interesting. Employing a spamfilter is obviously a necessity for mail to remain useful at all. But even with a filter assuming a sounds-nice false negative percentage of 1 (i.e. assuming 1% of the spam is not filtered) that's 20 emails for manual sorting every day. That about as much spam as I receive total and I'm annoyed. Weinberger needs to move into the next digit. Handling false positives, i.e. accepting any messages scored as spam by manually removing them from quarantine is mostly useless with this volume of spam, so in order to ensure reception of messages there is no way around some kind of challenge response like scheme. It may be done socially instead of automatically but it must be done.