While surely not all, a good number of ISPs already provide anti-virus and anti-malware detection. If you use an ISP for email, chances are pretty high that the back end is running anti-spam rules which looks for attachments and strips where a "bad" MD5 sum is found. "bad" meaning it matches malware/virused attachments. They don't need to read the contents, just look for the checksum.
As a privacy advocate I'm not too uncomfortable with ISPs scanning like this, as anything I would be worried about would be through a channel other than email which they could not scan.