goodroi - 11:11 am on Aug 18, 2011 (gmt 0)
Googlebot is just a computer program. Computer programs like when things are simple and easy for them. Whenever possible keep things simple.
If your out of stock products are going to be replenished in a few days I would not worry about it. Even if you made a more complicated sitemap to deal with out of stock items there is low probability that it would help you. Because Google would need to immediately see your sitemap, then prioritize googlebot to recrawl your deep product pages (which probably have no backlinks) then recrunch the Google index to deal with the out of stock pages. Then when you have the product in stock Google would have to repeat the complete process. The process of adding & dropping pages to Google typically takes many days/weeks.
If you out of stock items are going to be back in a few days, it is likely your page would be fixed before googlebot even gets a chance to revisit it.