In 3.1 Frost went from a far less preferred spec to one of the best, replacing Unholy altogether in many raiding Death Knight's books.
Frost - 17/51/3
This particular spec relies heavily on having the proper glyphs. In order to get the most out of it, you must have Glyph of Howling Blast and Glyph of Frost Strike. You don't use Death Coil in this build - rather, Frost Strike takes its place with more damage for less runic power cost. You also don't use Plague Strike at all, as the build only requires you to have Frost Fever up on your target(s). To pull this spec off, you open with Howling Blast, which when glyphed puts Frost Fever on all targets hit. You use Blood Strike twice to start the conversion to Death Runes, then you alternate Obliterate (which doesn't consume diseases) and Frost Strike for chaining massive hits back to back. When your Frost Fever wears off, you start the cycle over again. In addition to massive amounts of personal damage, this spec also provides a raid-wide buff in Improved Icy Talons that grants melee haste. AoE with this spec is a little bit trickier, since Blood Boil doesn't cause your Blood runes to convert to Death runes. However, there is no need to use Pestilence because Howling Blast will put the disease on all targets hit. The three points in Virulence is very importance since hitting with Howling Blast, a spell, is key to the rotation. The 17 points in Blood do nothing more than increase DPS. Bladed Armor should never be underestimated, as it is an attack power boost that directly scales with gear upgrades. By reaching deep into Blood we also gain a 5% critical strike gain on all abilities. Since we don't dual-wield with this spec, Two-handed weapon specialization gives an immense DPS bonus.