How to generate random boolean value in sql server 2008?

You can use

CRYPT_GEN_RANDOM(1) % 2

The advantages over RAND are that it is stronger cryptographically (you may not care) and that if inserting multiple rows it is re-evaluated for each row.

DECLARE @T TABLE(
  B1 BIT,
  B2 BIT);

INSERT INTO @T
SELECT TOP 10 CRYPT_GEN_RANDOM(1)%2,
              CAST(ROUND(RAND(), 0) AS BIT)
FROM   master..spt_values

SELECT *
FROM   @T 

would give the same value in all rows for the second column

If you are only generating one row, you could use something as simple as:

SELECT CAST(ROUND(RAND(),0) AS BIT)

However, if you are generating more than one row, RAND() will evaluate to the same value for every row, so please see Martin Smith's answer.