It has now been confirmed that Hail did not get this boost in Gen V. Hail is the same as it was before.
Battling in a hailstorm
Damages all Pokémon who are not Ice-type.
Does not damage Pokémon with the following abilities: Ice Body, Snow Cloak, Magic Guard, Overcoat.
Allows Blizzard to bypass accuracy check.
In Pokémon Diamond and Pearl, allows it to hit through Protect and Detect 25% of the time.
Causes SolarBeam to deal only half of its normal damage.
Causes Moonlight, Synthesis, and Morning Sun to recover ¼ of max HP.
Activates the abilities: Forecast, Ice Body, Snow Cloak.
Changes Weather Ball to an Ice-type move and doubles its power, and Castform to its Hail form.
Can be lengthened from 5 to 8 turns with the use of the Icy Rock.
Activating move: Hail
Activating ability: Snow Warning
Routes where it is encountered naturally: Route 216, 217, Mt. Coronet peak, Mt. Silver peak.