unnest in SELECT

I have a couple of questions regarding unnest.

1) If I issue a select statement "select unnest(vector1) as v from 
some_table", I cannot seem to use the column alias v in a WHERE or 
HAVING clause. I can use it in an ORDER BY or GROUP by clause. Is this 
the way it is supposed to work?

2) If I issue a select statement "select unnest(vector1) as v1, 
unnest(vector2) as v2 from some_table" and vector1 has a length of 3 and 
vector2 has a length of 4, the result set will have 12 rows with the 
data of vector1 repeating 4 times and vector2 repeating 3 times. 
Shouldn't the content of the shorter array(s) simply be return null in 
it's respective column and the result set be the size of the longest array?

Thanks,
Karl