The switch function from earlier versions is gone in 1.5 when viewing stats on your blog's dashboard. If you view stats from any WordPress.com dashboard you can still switch.
This is because the plugin now uses an API key to authenticate with WordPress.com. Earlier versions used an iframe, which meant your browser connected directly to WordPress.com and you had to be logged in to see the stats. As such, we could tell who was looking at the stats and show a list of other blogs belonging to that person. Using the API key, your blog can show stats to any Author or above. Therefore we don't know who is going to see the report.
If you like the switch function, use http://dashboard.wordpress.com/wp-admin/index.php?page=stats