- is the method name right? would attr_calculated or attr_cached be better?
- allow the column name to be customized
- if there's only one dependent association, allow it to not be in brackets
- allow :dependent to accept other attribute names, so that the calculated attribute is only re-cached if those change (requires a hash column)
- provide TTL option, so that attributes can auto-expire if they're over x seconds old. requires an updated_at column.
- attach before_validation callback to dependent associations to trigger recalculate_attributes(!)
- provide option so that merely reading an object could recalc-and-save if necessary.
- are 'calculated' and 'dependent' the best terms? perhaps 'cached' and 'depends_on'?

don't think so:
- allow an option to specify lazy-recalculation