Sony has just announced they have put missing PSP titles, such as Motorstorm Artic Edge and Everybody’s Tennis, back up on the PS Store, in Japan at least.
This only counts for Japan, but hopefully the rest of the World will soon see the return of these titles.
The titles had been taken down due to an exploit which may have allowed people to use Homebrew on PS Vita. Now it seems like this exploit has been addressed.
Source: Sony, via NeoGAF.