Recent studies by heise Security staff of several thousand valid certificates, none of which generated an error in a broswer, found that approximately one in 30 of these used weak keys - an alarmingly high number. Among these were online shops where people would be expected to enter their credit card details.

For a certificate to be accepted by a browser without issuing a warning, the certificate needs to be issued by a recognised certification authority (CA). All of those that we contacted said that they would revoke any weak keys and freely replace them, but it seems clear that not many certificate owners have checked and replaced their certificates.

More at:
http://www.heise-online.co.uk/news/Many-weak-web-server-certificates-threaten-online-shopping--/111023