OP - don't know that I'd use Tyvek for overboots but it would probably work as an internal liner for cold weather. Will have to give it a try.
Originally Posted by
scroca
No, but next time I ride by a construction site when it's raining, I'll keep it in mind.
No need for a construction site, Fedex and USPS use Tyvek mailer envelopes. I always save these and use them to wrap up stuff that goes in my panniers.