Yes, grass is great for rabbits, it's their main staple, although unless the rabbit is allowed to graze, it's hard to keep them properly supplied with it.
You can't cut it because it breaks down too fast, so you have to pick it and provide enough variety in terms of stages and types of growth, etc. They also like to munch on trigs and trees.
And it's nice to include some other vegetation that grows in the grass, but personally in my yard I'm not sure what the rabbits like or can eat, so again its hard to bring it in to them.
EDITED TO ADD: NEVER feed grass from an area where raccoons and other wildlife reside, parasites like raccoon roundworm are deadly to rabbits. The eggs survive in the soil for years and are very hard to wash off the leafs.
sas :bunnydance: