It's not exactly practical to do—what with different screen resolutions and such—but it's possible to run iPad apps on your jailbroken iPhone. And true, the idea isn't exactly new—enterprising folks did it when Netflix was iPad only—but it's a bit easier to do now. Here's how.
Basically, you'll need to download and install the iTelePad2Pod app (Mac only, download here) and have a jailbroken iPhone installed with OpenSSH (from Cydia). The iTelePad2Pod app changes the metadata of previously purchased iPad apps to trick it into thinking it works for iPhone. The .app file is then transferred to the iPhone via SFTP. It doesn't look too hard to do but isn't guaranteed to work for every iPad app (as some just won't work). More instructions on how to get this hack working can be found at ModMyi. [iJailbreakNow via ModMyi via 9to5Mac]