How to check existence of user-define table type in SQL Server 2008?

IF EXISTS (SELECT * FROM sys.types WHERE is_table_type = 1 AND name = 'MyType')
    --stuff

sys.types... they aren't schema-scoped objects so won't be in sys.objects

Update, Mar 2013

You can use TYPE_ID too

Following examples work for me, please note "is_user_defined" NOT "is_table_type"

IF TYPE_ID(N'idType') IS NULL
CREATE TYPE [dbo].[idType] FROM Bigint NOT NULL
go

IF not EXISTS (SELECT * FROM sys.types WHERE is_user_defined = 1 AND name = 'idType')
CREATE TYPE [dbo].[idType] FROM Bigint NOT NULL
go

IF EXISTS(SELECT 1 FROM sys.types WHERE name = 'Person' AND is_table_type = 1 AND schema_id = SCHEMA_ID('VAB'))
DROP TYPE VAB.Person;
go
CREATE TYPE VAB.Person AS TABLE
(    PersonID               INT
    ,FirstName              VARCHAR(255)
    ,MiddleName             VARCHAR(255)
    ,LastName               VARCHAR(255)
    ,PreferredName          VARCHAR(255)
);

You can look in sys.types or use TYPE_ID:

IF TYPE_ID(N'MyType') IS NULL ...

Just a precaution: using type_id won't verify that the type is a table type--just that a type by that name exists. Otherwise gbn's query is probably better.