Nate,
There are a couple of options here:
1. Disable pre-fill on the comments field.
2. Create a NEW comments field to avoid having Sales write in nasty comments that accidentally get displayed back to the lead. It does happen. I guess this is an issue more with the Description=Person Notes field in SFDC.
3. Use Comments History:
Option Comments is listed on your Form. This is a "most recent" field. Then create a new field in SFDC called "Option Comments History". Stop displaying the first field on the page, then put the history field on the page to Sales.
Then follow the instructions in this post:
http://www.marketingrockstarguides.com/marketo-hidden-tricks-tips-1482/