When building a multi picklist - where would you see all the values that someone has selected, would it show up in their contact view?
I would show up in the lead record in Marketo under the field name you uploaded to. It would look like the list in Josh's response above.
Multiple values will show up in the field in the Lead View and would look as Josh suggested:
Or you can add the field to your view.
FYI, using semicolons to separate the values can very tricky on some non english versions of MSOffice. In French, for instance, the default column separator for CSV files is the semicolon. As a result, one need to reprocess CSV files and replace semicolons with commas, but it your file has semicolons within field values, it completely breaks the import file, adding unwanted columns.
Of course, we can workaround this, but I just wanted to give a heads up for international companies.