Pressure cookers were introduced to North Africa during the 1970's. They were widely embraced for two very important reasons: dramatic reduction of cooking time and fuel use.
A dish prepared in a tagine is called a tagine. The same dish prepared in a pressure cooker is often called a marqa (sauce), for the simple reason that pressure cooking usually produces more sauce (although, you could reduce the sauce in a pressure cooker by removing the lid and turning up the heat).
My lamb tagine for the LA Times involves five hours of prep and cooking time. A dish like this would definately call for a pressure cooker on weekdays when I am working or I would save it for slow cooking in a tagine on a lazy weekend.
Note that my recipe in the Times calls for browning the lambshanks because I am using a large stew pot to cook the tagine. I brown meat first when I use a pressure cooker. If I were cooking it in a clay tagine, I wouldn't brown the shanks (more later about why).
Converting tagine recipes for pressure cookers is pretty simple: brown meats or poultry first; add vegetables in order of cooking; remove cover towards the end of cooking and turn up heat to reduce sauce if desired.
Dishes like my seafood tagine take so little time to cook that pressure cooking is unecessary.