18 months on... I get this problem too. What's happening is that when the post comes from Delicious, and it is published, the entry appears without category.
When you go in the interface, the category is set so when a republish is done, all is fine.
I can't for the life of me work out why this is happening - and I
REALLY want a solution because this affects
EVERYTHING where you use a category.
For example, I do a detect and overwrite the EntryExcerpt field for anything in my delicious links category in all manner of places (so catentries is no use to me!)
The only solutions I can think of are to use dynamic publishing (as I use sqlite, I really don't want to do this), or to use the
mt-rebuild plugin and schedule a cron job to do a rebuild of the entire site five minutes after delicious is supposed to have published (which is what I'm planning on doing).
Both are of course hacks which don't solve the underlying problem...