No, backup will not truncate the log, nor it will shrink the log file.

This will shrink the log file if conditions from my previous post are met:

-- find logical name of the log file

sp_helpdb 'MyDbName'

-- Shrink the log to 1000 MB

CHECKPOINT

DBCC SHRINKFILE('MyTransactionLog', 1000)

This could also be useful:

-- log file size and percentage free

DBCC SQLPERF (LOGSPACE)

-- Why the log is not truncated

select name, log_reuse_wait_desc, recovery_model_desc from sys.databases

-- oldest active transaction

DBCC OPENTRAN

HTH,

Vedran