This is really, really cool and brings juju into a new area of capability for deploying big and little sites.
To be clear, this allows you to abstract things pretty effectively.
juju deploy mysql --constraints mem=10G
juju deploy statusnet --constraints cpu=1
This will result in your mysql service being on an extra large instance since it has 15GB of RAM. Your statusnet instances will be m1.small's since that will have just 1 ECU.
Even cooler than this is now if you want a mysql slave in a different availability zone:
juju deploy mysql --constraints ec2-zone=a mysql-a
juju deploy mysql --constraints ec2-zone=b mysql-b
juju add-relation mysql-a:master mysql-b:slave
juju add-relation statusnet mysql-a
Now if mysql-a goes down
juju remove-relation statusnet mysql-a
juju add-relation statusnet mysql-b
Much and more is possible, but this really does make juju even more compelling as a tool for simple, easy deployment. Edit: fixed ec2-zone to be the single character, per William's feedback.