The Treaty specifies the conditions for participating in monetary union. They include, in particular, proof of a high degree of sustainable convergence (Article 121 (1) of the EC Treaty). That entails a high degree of price stability, a convergence of long-term interest rates, a lastingly sustainable government financial position and exchange-rate stability.
This raises the question of whether or not central banks should delay acting if circumstances change, in order to placate the markets. I would suggest not. While we do not want to surprise the markets, if we believe action to be appropriate, we need to act. It is always more difficult to signal the beginning of a cycle.