VACUUM FULL and ALTER TABLE do *nothing* in 7.3.2

Dear Gurus,

I have a problem I've never met before, nor similar. Also, I don't have any
ideas how to search the archives for similar accidents. If you can point me
to the right direction, please don't hesitate to do so.

I have two tables and I wish to make a FK between them, such as:
 ALTER TABLE invoice ADD FOREIGN KEY (seller_account) REFERENCES account(id);

Quite simple, isn't it? In 7.2.1, it executed in a second or two for two
db's (essentially, the same skeleton with different data. No big deal).
However, in 7.3.2, it does __nothing__ (not even a notice or warning or
info), or at least, it takes ages. psql won't give the prompt back in 20
minutes (yes, minutes... still running...)

Tried this and tried that, and finally tried
 VACUUM FULL VERBOSE ANALYZE invoice;

It did *nothing* though I waited for at least two minutes. Not even the
first INFO line, as in the output for VACUUM VERBOSE ANALYZE:
 INFO:  --Relation public.invoice-- INFO:  Pages 44: Changed 0, Empty 0; Tup 1318: Vac 0, Keep 0, UnUsed 127.
TotalCPU 0.00s/0.00u sec elapsed 0.00 sec. INFO:  --Relation pg_toast.pg_toast_3349048-- INFO:  Pages 0: Changed 0,
Empty0; Tup 0: Vac 0, Keep 0, UnUsed 0.         Total CPU 0.00s/0.00u sec elapsed 0.00 sec. INFO:  Analyzing
public.invoiceVACUUM
 

As you may have found out, VACUUM ANALYZE and ANALYZE seemed to work, but
VACUUM FULL ANALYZE and ALTER TABLE didn't.

What may be the problem? If ps output helps:

17058     1  0 00:00:04 /usr/local/pgsql-7.3.2/bin/postmaster                                               -p 5433 -i
-N128 -B 256
 
17060 17058  0 00:00:00 [postmaster]7324 17058  0 00:00:01 postgres: sd bin 192.168.3.10 idle7325 17058  0 00:00:04
postgres:sd bin 192.168.3.10 idle in transaction
 
17238 17058  0 00:00:09 postgres: postgres bin [local] ALTER TABLE waiting

... and CPU is around 10%. What is this "waiting" thing?

G.
--
while (!asleep()) sheep++;

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