how difficult would it be to be able to place an order for items which are 'out of stock' (since your products aren't something like many pinball parts, where once they're gone, they're gone) and to:
a) offer the ability to either ship out what is in stock immediately with the balance shipping out once it becomes available again or
b) wait and ship the entire order once the 'out of stock' items are re-stocked?
a few companies that I order things from offer this ability (where the items are pre-order goods and the release dates between them is a month apart or less).
from a consumer's point of view, it might make things a bit easier than for say having me order a portion of what I need and then to still keep checking the site to see if the 1 or 2 items have come back in stock and then order them.
just a thought