I just did a fresh scan with ClamAV, and it looks like they've cleared the false positive:
$ clamscan optipng.exe
optipng.exe: OK
----------- SCAN SUMMARY -----------
Known viruses: 4230664
Engine version: 0.98.7
Scanned directories: 0
Scanned files: 1
Infected files: 0
Data scanned: 0.09 MB
Data read: 0.09 MB (ratio 1.00:1)
Time: 7.338 sec (0 m 7 s)
Thanks for hanging in there everybody! Webhosts should have updated definitions soon, so we can get back to real business.