Anybody who checks in to your venue or visits your foursquare.com venue page will see your Update.
As for the folks that Foursquare identifies as your customers (see below), they will receive your Update as soon as you send it out in their Friend Feed. Your Update will appear right alongside their friends’ check-ins. This is very meaningful placement and a great way to get your messages out to your customers in a way that they are sure to notice.
Your “Customers” are anyone who... - likes your venue(s) - likes your Foursquare page - Checks in at your venues a number of times
Remember, every user will see your Updates on check-ins and your venue pages!