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]