[development] Temporary form data storage ... and avoiding validation

[Jeff Greenberg]

I'm using a module-generated form in a mode like a shopping cart, where 
the user can add data to it, go to other pages, and come back to it, and 
the data persists.

At first I wasn't submitting the data until it was time to complete the 
process, and so, to persist the data, I was using variable_get/set 
because without the form being submitted, nothing was being persisted in 
form_state.  However, I've since switched to each of the buttons (return 
to other pages, update form, submit) causing a submit, with varying 
logic based on which button was clicked. I discovered (duh!) that 
variable_set/get was a poor idea, because two users online at the same 
time end up would get each other's persisted data on the form.

So, two questions. Is form_state going to persist the user data for me 
between form loads, or is there a better method? And I've run into an 
interesting problem in that the portion of the form with required fields 
(name, etc) doesn't need to be filled in until the user wants to submit, 
but of course, that ends up throwing an error when the form is 
'submitted' due to one of the other buttons being clicked. Is there a 
way to have those buttons cause a submit, but bypass validation?