After hackers found exploits in Hot Shots Tennis: Get a Grip and Motorstorm Arctic Edge, Sony pulled both PSP titles from PlayStation Network. Both games had holes that let users run homebrew applications on PlayStation Vita via the PSP emulator. When the titles were removed even users who purchased the games were not able to download them.
While Sony did not comment on the exploit, it appears they patched the titles. Hot Shots Tennis: Get a Grip and Motorstorm Arctic Edge are available on PlayStation Network today, a Sony Computer Entertainment Japan statement explains.