Although there is no concrete answer, one can make an implication based on the events of "The Pegasus:"
1. Thanks to the TOA, Federation ships cannot have cloaking devices. This seems to be a very big deal (at least to Pressman, who felt that this held Starfleet back for decades).
2. Picard also seems to imply that it would be a very big deal if the Romulans found out about the Pegasus's cloaking device, and that it could lead to a new conflict between the Federation and the Empire.
3. So why is this all such a big deal? Well, if it is a situation of history repeating itself, the implication is that sometime in the past, the Federation was using a ship or ships with a cloaking device, and it pissed off the Romulans so much that there was an incident about it where lives were lost (Tomed). If the Federation freely gave up using cloaking devices after that, then it can be implied that that ship or ships were doing something they shouldn't by using those devices; something that caused the Feds to get egg on their face because of it. YMMV.