Enroute Controller described a loss of separation event when applying visual separation and the pilot's response not in total accordance with requirements; the reporter suggesting that flight crews should be more aware of ATC separation mandates.

Narrative

An E145 was at 110 and switched to TRACON; a B737 descending out of 130 to cross at 090. Based on the speed of the E145 it appeared TRACON reduced the speed below 250 KTS which allowed the B737 to catch the E145 on the back side while passing through 110. Prior to the B737 passing 120; I informed him of the E145. Pilot reported aircraft in site and I issued instructions to maintain visual separation. Pilot read back 'ok I have him in site'. I restated the need for him to read back he will maintain visual separation. Pilot read back 'I will maintain visual' and didn't read back call sign. At this point separation was lost and I told pilot we need certain phraseology. I then switched communication of the B737 to TRACON. The simplification of visual separation acceptance by the pilot. Pilots are NOT informed the proper phraseology to accept visual separation. The FAA requires a specific read back and pilots are not held accountable for this phraseology. The FAA does not educate or insure accountability of the pilots. This has been a problem since the inception of visual separation in the en route environment. The requirements by the FAA for pilot read back needs to change or the FAA needs to take the time to educate proper phraseology to the pilots.