Me thinks that your supply voltage routing is a bit too fast and by this prone to resonances? SMPS can generate unsane narrow voltage spikes, which when not being sufficiently filtered out can then cause lots of trouble (ringing!).

I would increase the 1nF caps, add some ferrites or low ohmic resistors in the supply lines and use a post filter at the SMPS.
By the way, I never use a switcher to directly power the digital section, but always a linear 5V regulator (LM7805 type). Switchers I only use for pre-regulating, if needed.

I think, it's better to have a clean, spikes-free but smooth power-up, than a fast but ringing one. In any case, I know, that the DS1232 has lots of problems (even total lock-up!) with fast spikes on Vcc. The MAX1232 can handle them much better. But the best is to avoid these fast spikes and too steep edges at power-up.

Kai