Quoted from vid1900:They are much less likely to leak than alkaline batteries, but they do occasionally leak or vent.
The only batteries I can't seem to get to leak are the Sanyo Eneloop.
The guys on the Canon photography forums worship them.
They do finally die after about 7 years, but there does not seem to be anything inside them to leak out:
eneloop-battery-unrolled.jpg
Hey Vid, Are those the rechargeable batteries? The Eneloop ones? Any AA charger work charging them?