This bug seems to have been fixed in Snow Leopard.
My bug report to Apple:
Calculator does not accept "." input after conversion from the "Convert" menu.
Steps to reproduce:
1- enter a value with a decimal point, the decimal point can be entered either from the keyboard or from the the Calculator UI.
2- select any conversion from the Convert menu
3- the conversion result is displayed
4- enter a value with a decimal point again
The new value should have a decimal point in the location where it has been input
The decimal point cannot be input, neither from the keyboard nor from the Calculator UI. It is necessary to hit "C" on the Calculator UI to reinitialize Calculator to be able again to input a decimal point.