In Python strings, lists and tuples are all sequences and indexing operates on sequences. The above is leading to doing in Groovy what is done in Python. However, in Python, lists are mutable whereas strings and tuples are immutable. This means it is can be very troublesome writing supposedly polymorphic functions over sequences since there is no static typing to support overloading and you often have to distinguish lists, tuples and strings since the operation set is not the same.
My point: Although strings and lists appears to both be sequences, the boundary has to be drawn somewhere, the question is where.
In this case, at least in Java 5.0 and higher, the in operator could be mapped to the use of the contains method. I would therefore propose that for this example lists and strings can be treated polymorphically and so it would be good to do so. I therefore support making this feature work.
So, I suggest that we leave this open and when Groovy commits to Java 5.0 and above rather than the current Java 1.4 and above, this feature can be added. As Java 1.4 does not have contains in String, I propose no action currently.