ODK Build has been updated with a feature many of you have asked for in different variations: the ability to be able to save forms in a native format (an .odkbuild file, not an xform .xml file), and reload them into Build at a later time, largely for purposes of sharing forms with each other over email.
You'll find the two new menu options under the File menu. Because of the way the feature works (a form loaded from a file is treated as a brand new form, rather than resuming from the one you had saved in Build itself, to reduce confusion), you'll end up with a lot of duplicate files while swapping back and forth — by next week you'll have the ability to clear out your old forms.
Also next week will be the other feature I promised for this week, which was the ability to more easily and flexibly edit select one and select many prompts. I'm still working on that one; my day job building data publishing tools has interfered enough with my night job building data collection tools enough that I've had to slip the schedule a bit. Thanks for all your patience! =)
Please let me know if you see anything wrong since the update.