(When) is hash table lookup O(1)?

It is often said that hash table lookup operates in constant time: you compute the hash value, which gives you an index for an array lookup. Yet this ignores collisions; in the worst case, every item happens to land in the same bucket and the lookup time becomes linear ($\Theta(n)$). Are there conditions on the data that can make hash table lookup truly $O(1)$? Is that only on average, or can a hash table have $O(1)$ worst case lookup? Note: I'm coming from a programmer's perspective here; when I store data in a hash table, it's almost always strings or some composite data structures, and the data changes during the lifetime of the hash table. So while I appreciate answers about perfect hashes, they're cute but anecdotal and not practical from my point of view. P.S. Follow-up: For what kind of data are hash table operations O(1)?