A few people have tinkered with running JTS in GAE (eg here and here). But the killer app has yet to emerge... How about a Java-based App Engine version of this?
An example: Integration Middleware Removal
3 days ago
select ID, GEOM from T1 as t
HATBOX_MBR_INTERSECTS_ENV('PUBLIC','T1',145.05,145.25,-37.25,-37.05) as i
on t.ID = i.HATBOX_JOIN_ID