Ukitake's leaving the way that Ishida is introduced up to him, not ignoring him.
He thinks Ishida might want to remain an unknown quantity or use an assumed name if he assumes that the arrancar don't have anything on him. Pretty futile if you ask me, since Aizen takes Ichigo's friends more seriously than anybody else, but there we are. There was no way he'd be spying on two current and one past shinigami taichou without having some idea of who they were.
EDIT: And I was assuming this is before they know about any of the King's Key business, and therefore probably before any arrancar come to Karakura. That's why Ukitake and Shunsui are trying, in their rather unorthodox way, to find out a bit about their enemy. Urahara and Isshin are probably the only shinigami who would ever think they had a hope in hell of accomplishing anything this way.
*~Pips~*