Offense jumps from out-of-bounds to in-bounds, catches the disc while in the air. Turnover?

Short answer: yes.

Rule IX.C:
A player contacting the out-of-bounds area is out-of-bounds. A player who is not out-of-bounds is in-bounds. An airborne player retains in-bounds or out-of-bounds status until that player contacts the playing field or the out-of-bounds area.

-and-

Rule IX.E:
A disc becomes out-of-bounds when it first contacts the out-of-bounds area, contacts an out-of-bounds offensive player, or is caught by an out-of-bounds defensive player.

Although Ultimate etiquette and spirit of the game should stop a player from doing this, there is no rule stopping a player from running out-of-bounds all they want (except on a pull). Offensive players just need to make sure they come back in bounds before catching the disc!

Stay frosty,

Jarrod