Question : The transaction log for database 'mydatabase' is full. To find out why space in the log cannot be reused, see the log_reuse_wait_desc column in sys.databases

every time I get this error, at different points of testing inserts and deletions on my table:

The transaction log for database 'mydatabase' is full. To find out why space in the log cannot be reused, see the log_reuse_wait_desc column in sys.databases

Why do I keep getting this?  All I'm doing is deleting several hundred thousand records and inserting them into a couple of tables.  i shouldn't have to truncate my log every time or my application bombs out!

sys.databases only gives me this info for log_reuse_wait_desc which does nothing for me:

LOG_BACKUP

Answer : The transaction log for database 'mydatabase' is full. To find out why space in the log cannot be reused, see the log_reuse_wait_desc column in sys.databases

Check this

http://www.support.microsoft.com/?id=317375

As you say thousands of records get deleted and then get re-inserted in various table...just make sure that all these activities are in Beging and Commit trans block, as this might happen when there are large amunt of transactions are in uncommited stage so thoes will be lying in the transaction log which will keep increasing the transaction log size.