There will be a normal release cycle: beta, release candidate, final. I cannot give dates on when 1.0 will go final at this time. It will depend on how fast the bugs are knocked out. Getting 1.0 final is the highest priority right now and we are working to get there as fast as possible. If you want to avoid API changes, waiting for rc1 would be the best bet before beginning a upgrade.
I will be posting a roadmap in the next few days that will detail the upcoming releases and included functionality.
1.0 will go final when it is complete, independent of GWT 1.5 being released. Hopefully, GWT 1.5 will have been released by the time Ext GWT 1.0 is ready.