Actually, pressing delete just adds a transaction to the database saying "this item has been deleted", similar to the ones saying "this item has been completed" or "this item has a due date of May 7, 2011, 3:37 PM". It doesn't actually delete anything until your database is compacted. Until that point, one could still recover a deleted item, and if you were deleting something to get rid of its footprint (a large attachment, for example), it won't actually reclaim the space.
I believe that if you are not syncing OmniFocus to another device (or Mac), database compaction only happens when you request it with File->Compact Database. If you are syncing, the compaction happens automatically when all of your devices are in sync and there are transactions more than an hour old (transactions less than an hour old are not compacted). If you've deleted something and want to get it back, call up Omni and don't let your devices sync until they've helped you!