I've had a manuscript at an LDS house for quite some time. I haven't heard anything on it since November 2005. Would it be acceptable to contact the publisher after the convention and inquire on the status of my manuscript or keep waiting? Since I've already broken the rule of sending this same publisher a second manuscript (wish I'd had your advice before I did that) I don't want to make another mistake, but I'd like to know if my manuscript is still under consideration.
They've had it 9 months? You can grow a baby in that amount of time! No way should a submission review take that long. Yes, give them a couple of weeks to get through the convention, and then call and politely ask the status of your manuscript. You've been patient enough.
Unless their website says their normal submission process takes 9 months (and if it does, rethink submitting to them), anything more than 3 months on a basic query, or 6 months on a full, is a sign that something has gone wrong--either it never made it to them, or it's lost on someone's desk, or your rejection is lost in the mail. And I say rejection, because if they were going to accept, they would probably call you.
In the future, when you submit, make a note of what the publisher says is their usual response time. You can usually find this on their website. Then add 3 weeks. If you haven't heard from them by that date, then by all means, you may call and inquire.