Upstream is usable, but I figure most people would prefer a Slingbox because it avoids a VPN.
This thread has some technical details.
After starting to write out directions, I don't feel this is for the faint of heart, because it could break anytime Tivo updates the app, and then you'd have to know some networking to figure out what's wrong.
Three methods to VPN from IOS when you're out and about.
1) Built in support, like so: http://support.apple.com/kb/ht1288
2) 3rd party software that require jailbreaking
3) External wifi hardware (hotspots, laptops, smart phones in connection sharing mode)