I'm not totally sure of it, but I do think that Tommy and Lil are a popular couple on AGU and on the regular Rugrats.... (mostly on AGU, though ;) ) Anywho, here is some evidence that i've collected over both Series ^_^
All Grown Up
tommy ends up with Lil at the valentines dance, and walks with her on the dance floor, hand in hand.
They Sleep next to each other when they have a sleep over for studying for tests.
He always seems to be next to her ^_^...!
She Winked at him one episode
The same episode, they work next to each other.
Rugrats
One Episode, they were role playing some detective game, and Lil flirted with tommy... [lol, warm milk gets him every time...]
He was helping Lil fight off Angelica for one of Taffy's Hair clips.
Tommy went into a cave full of Lizards [immagination sequence] so he can find Lil's ring... (I loved how he risked his fear for her, so she can get her ring!... awww...)
She saw him naked numerous times, and didn't seem to care... [LOL]