I was one of the 3,521 who received the email with all 46,000 addresses in the CC field.
It was followed up by an apologetic email explaining what had happened and asking me to delete the original email; and another email sent to all 46,000, again apologising and explaining, and linking to the press release. The Register also promptly reported themselves to the ICO.
My first question to them was 'What mass mailing software or service do you use, and why did it allow this?' Considering the (assumed) IT literacy of The Register's staff, I hope they're at least using some semi-competent service, and not some home-brewed PHP script running a SQL query. They haven't answered this yet.
However, at least they have been honest and apologetic about it - this isn't the first time I've received mass CC'd emails from legitimate businesses, and usually they're a) unaware of it and b) don't really care even when I point it out. Always surprising considering how tight data protection laws are in Europe. (Maybe it's one of those pesky interfering European laws that the Tories want to get rid of, to let good old honest British spammers return to their traditional methods)
Anyway, I haven't deleted the email yet, but will be soon. While I have NO intention of sharing or using the 46k addresses (and have been impressed so far with the lack of replies from any of the other 3,521 along the lines of 'LOLZ!! EL REG FAIL!'), I might do some data analysis (eg. how many use each popular email service, what proportion use throw-away or site-specific addresses, how many apparently use real names) - any suggestions?