Current value of a sequence?

I'm trying to find the next value that will be assigned by a sequence
without actually incrementing it (yes, I know it isn't atomic-safe
and all)

I had a lot of trouble finding information about sequences in
the current documentation (maybe it is in the A4 PDF version or
something). When I finally resorted to trolling through the
source code, and found currval, it seems to have an odd (IMHO)
behavior:

zipcode=# select currval('messages_pkey_seq');
ERROR: messages_pkey_seq.currval is not yet defined in this session

...however, if I select nextval() first, it works. Is this
lazy caching? Is there a good way to find the current value of
sequence without querying directly (and possibly breaking in
future implementations?)

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