[tenuousemphasis]

>In the past few months, things appear to have changed, and emails that are obviously spam, complete with the misspelled titles and funky case and punctuation, are now appearing in my Gmail inbox. It looks like the Gmail spam filter is becoming less effective.

Yes, I've noticed the same thing. Emails that are quite obviously spam (multiple recipients, complete gibberish, PDF attachments) are getting through the Gmail spam filter.