People talk about publishers building their lists at certain times of the year...as in, "we already have too much YA Paranormal on our fall list."
Are there ideal times of year for a MS to go on submission in the sense that publishers acquire on a specific schedule?
Not really. Ideally, you don't want to send things out in August or December if it can be helped, cause everyone is on vacay. Other than that - no.
Basically, things are acquired a year or more before they'd possibly come out, and the release date is based on many factors: How many other things are already waiting in line, how much work the item in question will need, if there has to be an illustrator involved, when the author has other books being released, etc... Like, if there is an illustrator, his schedule has to be taken into account too. If the author has other books, you wouldn't want them to conflict. If there are a ton of picture books coming in the upcoming season, you might want to add more novels to balance it out. If somebody has been going wild on the fantasy series and they notice that the catalogue is looking too full of that, they might want to even it with more realistic fare. Etc etc.
So at the moment I am selling things for Fall '10, Spring '11 and Summer '11 - but then I just sold something for Spring '12. When I submit it has no real bearing on when it will be published (except that it will rarely be sooner than a year out -- at this point, it will be very unlikely to be in Spring '10 unless it is an emergency rush-job, because editors are trying to finalize Spring '10 books now.)