> Sum = Struct.new(:bla, :"sum(value)")
> sum = Sum.new(2, 5)
=> #<struct Sum bla=2, :"sum(value)"=5>
> sum.send(:"sum(value)")
NoMethodError: undefined method `sum(value)' for #<struct Sum bla=2, :"sum(value)"=5>
> class X
>   attr_accessor :"sum(value)"
> end
NameError: invalid attribute name `sum(value)'
> class Sum
>   define_method(:"sum(value)") { "summum!" }
> end
=> #<Proc:0xa7c5f218@(irb):181>
> sum.send(:"sum(value)")
=> "summum!"

Not sure this is an error in ruby (1.8.5) or a misunderstanding from me (about Struct).
And yes, I know how to get/set the value anyway:

> sum[:"sum(value)"]
=> 5
> sum["sum(value)"] = 42
=> 42

Bye,
Kero.