Marketo has the ability to see and pull data from Salesforce Formula fields, there is however a catch which will be explained in this article.
Everytime the Marketo Sync connects to Salesforce it will scan records and look at the "SytemModStamp" (salesforce system field) for each one of them. It will compare this value with the stored value, which was pulled at the last scheduled sync. If the values match, Marketo will move on to the next record. If the values are different (new value later date than previous value), then Marketo will do a compare and contrast of all fields on that record in both systems and update the information as needed.
When a normal non-formula field is updated and changed on a Lead/Contact record in SFDC, the SytemModStamp value is updated. This is how on next sync Marketo knows to do a compare/contrast check and pull updates. Formula fields do not behave the same way. A formula field is calculated based on data in fields called upon in the formula; this means that the formula field calculation itself will not update the SytemModStamp in Salesforce.
Chances are you already have existing records in SFDC and Marketo. If you were to create a formula field today in your instance of SFDC and have it sync down into Marketo, the data calculated for the formula field in SFDC will not come into Marketo right away. The reason for this is, the formula field has created data based on already existing data, this does not result in a SytemModStamp change.
Typically formula fields will be a calculation of data from fields which are somehow related to the lead/contact record. This means that moving forward, any change in the normal field, will result in a SytemModStamp change as well as a recalculation of the formula field. In this case, Marketo will see the updated SytemModStamp due to the normal field change. Marketo will do the compare/contrast excercise and find that the formula field also needs updating.
If you create a formula field in SFDC and would like to have all the historical data for the formula field to come into Marketo, you can force an update on the records in SFDC to update the SytemModStamp. This way, on next sync, Marketo will see the formula data and pull it in. Alternatively, you can simply allow for natural SytemModStamp updates in SFDC to occur which should result in a slow trickle of historical data from SFDC into Marketo for the newly created formula field.