AHA! I CALLED IT! EVERYONE ELSE WAS SURPRISED BUT I SAW IT COMING! IT. IS. CANON!

The best evidence I have is when they met. Sakura and Naruto were trying to teach Sai to be more social. One of the things they tell him is that he should give nicknames to people... He responded by giving Sakura the nickname "ugly". Cue Naruto trying his best to keep Sai from a cruel bloody death. They decided to explain to him he should give nicknames that don't go with someone's personality. One arrives and Sai wants to test it on her. He marches right up to Ino and says "Hello, beautiful". Yes, he basically insulted Ino, but Ino didn't know that. And the blushing! She quickly develops a crush on him, and the rest is history.