Microsoft released Exchange 2007 Update Rollup 1 this month, available at http://support.microsoft.com/?kbid=930809. The Exchange team said Rollups would appear every 6 to 8 weeks. Exchange 2007 Rollup 1 isn't quite a Service Pack -- meaning it doesn't include any new functionality. However, the quality and amount of testing is far superior to traditional hotfixes.
This 29MB(!) package includes fixes for Public Folder replication and backups. The update fixed an issue I was having where my Exchange backups were failing because the VSS provider was taking too long.
It's interesting to note that the update takes several minutes to prepare itself for installation, even after it offers to let you continue. The update itself took about 15 minutes to apply. Just be patient.