[nodamage]

> If I try to do an UPDATE ... SET x = x + 1, that will always increment correctly in SQL. But if read x from an ORM object and write back x + 1, that looks like I'm just writing a constant, right?

This is not specific to ORMs... you can run into the same problem without one.

> Extra magic: if you've read a class from the db, pass it around, and then modify a field in that class, will that perform a db update: now? later? never?

In every ORM I've used you have specific control over when this happens.