Ablative armour was originally developed for the USS Defiant1; the Borg had proven their ability to penetrate Federation shields with ease during their encounter with the Enterprise at system J-252, and the Defiant designers wanted the ship to have a degree of protection even if the shields should be overcome in the expected future encounters.3
The value of the armour was proven when the Defiant came up against Dominion forces. Like the Borg, the Dominion were able to penetrate Federation shielding systems relatively easily in the first years of hostility between the two powers4, and the Defiant's ablative armour allowed the ship to withstand attacks that would have otherwise caused significant damage.5
Recently there has been a massive revolution in this technology with the appearance of the ablative armour generator. This device was given to the crew of the USS Voyager by a future version of Admiral Janeway, who travelled back in time to the ship in order to assist them in returning home from the Delta Quadrant. The generator involves a series of devices placed onto the hull of a vessel; when activated these replicate a layer of armour over the surface of the ship. The effectiveness of armour is considerable. The system Voyager employed was able to withstand a simultaneous attack by several Borg cubes. One of the most useful features of this system is that as it can be materialised and dematerialised as needed, even weapons systems can be covered over when they are not actually in use. This gives a much greater degree of coverage than any standard armour protection which must leave permanent apetures to operate such systems through.6
At time of writing Starfleet had had relatively little time to asses this technology, but it was simple to adapt it for use on Voyager without spacedock or any outside support6, indicating that it should be reasonably easy for the remainder of Starfleet's ships to create ablative armour generators for themselves. This should multiply the hull strength of the fleet massively at a single stroke.