David or Eunice Boucher wrote:
[...]

a.  How does Boinc decide that you're ready to receive more work units?

b.  How does it allocate between climatepredicition workunits, which can take up to a year to process, versus seti work units, that can be finished in less than a day?

See:
http://boinc.berkeley.edu/intro.php
http://boinc.berkeley.edu/work_distribution.php

In short, Boinc is quite intelligent at juggling whatever cache range (days work) you've set, WU deadlines and expected completion rate.

Clever stuff.

Good luck,
Martin