How to check date-interval constraints

Hi,

I'd like to have a table that looks like this:

my_option (
id serial primary key,
myvalue double,
valid_start timestamp,
valid_stop timestamp
);

I want to store values that are only valid in a given
start-stop-interval so I could find a date-specific value for NOW()
or some other given date.
select myvalue from my_option where somedate between valid_start and
valid_stop;

How can I have a constraint, that prohibits nesting or overlapping
intervals?

1 7 2006-1-1 2006-1-31
2 9 2006-2-1 2006-2-28 OK
3 5 2006-1-10 2006-1-20 BAD lies within line 1
4 3 2006-1-20 2006-2-10 BAD starts within line 1
and ends in line 2

To make it even more interesting, it'd be nice to add a type-column so I
could ask:
select myvalue from my_option where now() between valid_start and
valid_stop AND mytype=42;

Then interval should ONLY not overlap with other intervals of the SAME type.