All existing plugins made for 1.6 should work fine in 1.7.
Similarly, developing anything "for 1.7" should be backwards-compatible - although there are several new functions etc I don't think they are things that would be used in plugins.
I think the biggest change that might affect people is the new "ranking layout" for the Users and Tags pages. It replaces the tables with divs similar to Stack Overflow. By default it's turned off for backwards-compatibility.
Anyway I think the time to look at plugin/theme development will be when the 1.7 beta is released (in the next couple of weeks).