Did Nasus get a significant buff or did people just sort of realize that whither is completely ridiculous?
His Q got a significant buff, because it now gives you 2 stacks instead of 1 when used to finish off a large jungle creep (big wraith, big wolf, etc). His Q also now has a flat mana cost of 20 instead of 20/25/30/35/40, and while he is ulted his Wither (and all spells) have a range increase of 100. As you can imagine, having an 800 range targeted 90% slow is quite beneficial to the whole ganking thing, and being able to get more Q farm in the jungle as well as overall having more jungle sustain makes him a pretty good choice.
He has no where near the potential of Lane Nasus, but it's not uncommon to run him in a team comp that has say a Top Chogath on it so that they can eventually switch roles, with the Chogath doing jungle stuff once Nasus has the items and momentum to be immovable in lane. Or Nasus can stay in the jungle if he's doing fine, since he doesn't need that much gold to build tank and still do damage in fights.
It also should be mentioned that the Season 3 changes benefited him. Armor pen changes made his E better, itemization changes made it easier for him to get things he wants (in particular, Ice Born Gauntlet is fantastic on him in the later parts of the game), and while Life Steal on items have been nerfed a lot over time (Bloodthirster went from max 25% to max 18%, Vamp scepter went from 12 to 12, Wriggles used to be 18 and is now 10, etc) Nasus's lifesteal has been untouched since it was buffed to 14/17/20 from 10/15/20 back when Skarner was released. That 20% lifesteal is more than any one item will give you, outside of Sanguine Blade which hits 22% at full stacks.
So yeah, Nasus has been getting both direct and indirect buffs for a while now and people are finally starting to capitalize on that. --- http://i.imgur.com/7Wah6.jpg I'm actually incredibly overly passionate and...whats the word...viscous? - EltoniaX