The City is full of buzz about how a group of hot-shot dealers and sales people have been reneging on agreements with prospective employees. In some cases, they have actually signed contracts to start new jobs, but then walked away when better offers have come in. Of course, in the current environment, they can perhaps be forgiven for looking after themselves. But when the package they’ve been offered is actually extremely decent, it is surely unethical to renege because more dosh has been offered.
Ultimately, the FX market is a small community. If things go wrong for the hot shots, which given the history of forays into the market is a strong possibility, they are likely to find themselves short of friends. But I doubt they are bothered.