I was wondering if it was possible to launch an iPad app directly from within another app. For example, could I write a login page that takes you too a desktop type page wh
It depends on the app you wish to launch. Some of them register URLs that let you launch links like itunes:// and the like. Check this out: http://developer.apple.com/iphone/library/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/StandardBehaviors/StandardBehaviors.html#//apple_ref/doc/uid/TP40007072-CH4-SW7
The important section is the one entitled "Implementing Custom URL Schemes".
EDIT: Here's a link to a website that catalogs the various URLs for existing apps: http://handleopenurl.com/.