Something close to this will be in 3.1, so you'll want to remove the rule then, maybe name it something else too.
uri URI_SCHEME_MIXED_CASE /^(?![a-z]{3,6}:|[A-Z]{3,6})[A-Za-z]{3,6}:// describe URI_SCHEME_MIXED_CASE URI scheme has mixed uppercase and lowercase
The mass-check results are good, hits about 1% of spam with basically no false positives in 92,000 hams from 6 people.
Daniel