Actually this is (almost) all I wanted to say. This is intentionally posted with all related keywords in title, in the hope that a related search on Google will result with this post on first page.
I'm just still encountering companies who use MyISAM as their storage engine and are unaware that their nightly backup actually blocks their application, basically rendering their product unavailable for long minutes to hours on a nightly basis.
So this is posted as a warning for those who were not aware of this fact.
There is no hot (non blocking) backup for MyISAM. Closest would be file system snapshot, but even this requires flushing of tables, which may take a while to complete. If you must have a hot backup, then either use replication - and take the risk of the slave not being in complete sync with the master - or use another storage engine, i.e. InnoDB.