Indexing on arrays

Dear Hackers,

While working on a postgres-based fulltext searching system 
we encountered the following problem:
There is  a table  create table t (   x  int [] ) and a given integer constant y.The task is to find those records of
thistable, which contain thevalue y in the arrays x.
 

This could be implemented by writing a function array_contains(array,value)
and  selecting :select * from table where array_contains(table.x, y);

Such SQL statement would result in a long sequential scan, which is not
cool and not fast. It could be much more useful if we could use an index
for such a query.

If there were a kind of B-tree index which allows to have several
key values for a record, the problem could be  solved easily!

We would like to know if such a feature is already implemented          
in postgres indexes, otherwise  are there any serious difficulties in
implementing  it.

May be, GiSt could be useful for this task. Does anybody know any alive
implementation of GiST ?

Regards, Ivan Panchenko