It's a slow news day, so I thought I'd post this mod update for Battlefield: Apocalypse, which is perhaps the most promising PA mod being developed (it's the most frequently updated, at least). DoomSayer, their Lead Developer, has been interviewed at BF2news.net:
Cutlass (BF2 News): Will the mod debut with Battlefield 2 on June 27th?
DoomSayer: Unfortunately, it will not. We don't have access to the BF2 Editor yet. We need it in order to move from BFV to BF2. We also need to get hold of the game itself to see what all the code changes are. BF:A will most likely be released several months after BF2 hits retail stores. But on a positive note, this will also give players time to get a little burned out with BF2 and want something different.
That answers the question of how they're working on the mod without access to the game their mod is for.