Usually after the shipping date announcement, the last one gave people 2 weeks before the new OS started shipping.
So if it is similar this time, the OS will be announced mid-April (aka Tiger shipping announced April 12th 2005) and ship at the end of the month (Tiger ship date April 29, 2005).
Everyone who buys after the shipping date announcement (or gets a Leopardless machine after this date) would get a S&H upgrade under the up-to-date program (Tiger Up-to-Date was for purchases on or After 12th 2005).
Right, as I tried to clarify above, if the OS is announced before it ships, then Apple has a short grace period to take advantage of the "up-to-date" program. This program also always applies to Macs purchased at any time after the shipping announcement which came with the older version of the OS installed.