1) Am I correct that the username/password of my workers don't actually have to be "secure", as the most that one could do with them is submit shares for me, right? And they're passed unencrypted by the miner, right?
Yes, with worker login/password, nobody can do something wrong (change wallet, login to profile or so). But you still should keep this secret (I mean don't post it to forum or so), because somebody can sabotage your miner's work in this way.
Pool have memory for last 12 getwork requests per worker to validate submitted share later. So when somebody will request getworks using your worker credentials during your miner's work and your miner submit valid share, it can be rejected because attacker pushed out this job from pool queue already. So, nothing strange, but simply don't spread your credentials to other people.
2) Are there any plans for SSL for the web management interface? If not, it seems that an attacker could learn my account password (as opposed to a worker's password) or impersonate my session (as it seems to remember me via a cookie),
There is SSL enabled, but only with self-signed certificate. Currently I don't plan to change it to, because startssl.com offer only weak, 128bit certificates and classic certificates are quite expensive. But if you care, you can write down certificate fingerprint...
and then change the bitcoin address that rewards get sent to. If I'm vigilant I might notice, but an attacker may steal quite a few bitcoins from me before I notice. I do understand that I'm getting exactly what I'm paying for here, but as the pool becomes a bigger and bigger part of the bitcoin mining system, it may be a good plan to look at as it may start to become a target.
I agree that security IS the concern here. Firstly I was oriented mainly to security of pool algorithm, but it looks pretty good, so I can work on frontend improvements. Today I implemented CSRF protection, which improve security against javascript attacks.