A couple weeks ago, Google brought push Gmail to the iPhone via an expanded version of Google Sync. People were incredibly happy about this and they signed up right away.
Yesterday, people started reporting in a Google Mobile Help thread that Google Sync was down for them. The first reports came Tuesday late night or Wednesday early morning. Later on yesterday, Googler Bing, replied to the thread with a short heads up, "Thanks guys, we're looking into it!"
Soon after it began working again, for some. Others had issues getting things back in sync. Calendars, contacts and emails seemed to be missing or misinterpreted.
The thread is ongoing with complaints and issues, but I think, overall, Google has fixed the Google Sync issue by now.
Forum discussion at Google Mobile Help.