If you like Thai, Siam at 51st and Northern has some great options. Their vegetarian Tom Kha (sp?) is one of the best I've had because they load it up with some great al dente veggies in addition to the typical tofu and mushrooms: red bell pepper, baby corn, broccoli, cauliflower, snap peas, carrot, onion and more. I am not a vegetarian but I love it.
Siam Thai Cuisine
5008 W Northern Ave # 1, Glendale, AZ
Pita Jungle, a local chain of restaurants with a Mediterranean-inspired, semi-meatless menu has recently opened a location in the Arrowhead section of north Glendale. I haven't tried the Arrowhead location, but if it's as good as the original location in Tempe, it would be worthwhile. Favorites include the lentil fatoush salad and the cilantro-jalapeno hummus.
There are also a variety of Indian restaurants scattered in the various strip malls along Bell Road in both Glendale and northwest Phoenix. All would have substantial vegetarian offerings. I've been to a few of them, but not recently enough to be able to offer an opinion as to which are best.
Indian Maharaja Palace
5775 W Bell Rd Ste 5, Glendale, AZ 85308
Tandoori Times 2
5626 W Bell Rd Ste 103, Glendale, AZ 85308
Pita Jungle Arrowhead
7530 W Bell Rd Ste 106, Glendale, AZ 85308
5575 Windward Parkway, Alpharetta, GA 30004