JIRA 4.2 will have issue linking turned on by default and it will also have the following default link types:

clones / is cloned by
relates to / relates to
duplicates / is duplicated by
blocks / is blocked by

Note that these link types and the fact that issue linking is enabled are both only effected when starting JIRA from scratch with no data. If you upgrade, you don't get any change to your existing issue links or issue linking status, even if it is untouched from a prior default.