Constraining overlapping date ranges

PostgreSQLers,
I'm hoping for some help creating a constraint/key on a table such that there are no overlapping ranges of dates for
anyid.  

Specifically: Using PostgreSQL 9.0.1, I'm creating a name-value pair table as such this:

CREATE TABLE tbl (id INTEGER, start_date DATE, stop_date DATE, value REAL);

For a given id, I'd like to enforce that there is only one valid value on a given date. For instance, this would be
acceptable:

id    start_date    stop_date    value
2    2010-11-01    2010-12-01    3
2    2010-12-02    2010-12-15    4
3    2010-10-15    2010-12-15    -3

But this would not: (notice start_date of line 2 is before stop_date of line 1).
id    start_date    stop_date    value
2    2010-11-01    2010-12-01    3
2    2010-11-30    2010-12-15    4
3    2010-10-15    2010-12-15    -3

I'd also appreciate it if anyone can provide any indexing hints on this table to optimize queries like:
SELECT value FROM tbl WHERE id=2 and '2010-12-02' BETWEEN start_date AND stop_date;

Thanks in advance, and sorry if I overlooked any obvious documentation!

Robert McGehee