Over the next several weeks, they should be posting videos and other marketing materials about it. The depth of functionality in it remains to be seen. Based upon the description on their post, it sounds like it will be a basic Contact manager, as opposed to a complete CRM application. This is probably not much of a threat to Salesforce as a whole, but will likely be a viable option for single users and small teams that want to manage their contacts and manage activities related to them.
37 Signals has a tendency to release applications that people love to use (Basecamp for Project Management, Backpack for getting yourself organized, Backpack Calendar for calendaring, Campfire for chat), and they are leaders in the use of AJAX to enhance the user experience. Highrise will be an interesting application to check out. If nothing else, it should provide some user experience ideas to apply to Salesforce. Keep an eye on it.
You can sign up for their Highrise mailing list at http://www.highrisehq.com/.