As I know Checkpoint just writes dirty pages to disk, not at all the same thing.
Checkpoint is different and independent to commit/rollback.
This article demonstrates "http://technet.microsoft.com/en-gb/library/cc966500.aspx" (still Ok for SQL Server 2005 etc).
You can checkpoint and write to disk but subsequently rollback
The BEGIN/COMMIT is nothing to do with recovery model (full, simple, bulk logged). The TXN must either complete or fail as one.
In case of, say power failure, the data may on disk because of a
CHECKPOINT but uncommitted so is rolled back as part of the database
check this for more