Thanks, Cathal.
It seems to me like this would be a widespread issue for all of your customers who integrate with SFDC.
For now, I've disabled my SFDC sync in Marketo to prevent additional leads from being created until this is resolved.
My case was also escalated on the SFDC side (we have Premier support, so hoping for quick resolution there):
After reviewing your case, and seeing that this is an issue with the SQL syntax, I have forwarded your case over to our Developer Team to assist you further with this issue.
Latest from my Marketo case:
We only have limited auditing for the bi-directional sync but looking at the sync error log, this message appears to have started last night at the 9:56pm EST sync and has continued since then. This error message is something being provided by Salesforce as something that is occurring on the Salesforce side which leads me to believe there is an issue within Salesforce as that is also where the Leads are being created initially and then syncing to Marketo. We have had other clients call in with a similar issue since we spoke and we are performing some due diligence to verify whether there is anything we can do from Marketo to address this.