My daughter is 17 months and not walking yet. She qualifies for and recieves physical therapy from the state once per week.
When we went to our 12 month well-child checkup, her ped asked me lots of questions about is she doing this, is she doing that, and she pretty much wasn't doing any of it. At her 9 month check up, we had had no concerns though. At 12 months, she was getting around mostly by creeping on her tummy. She could crawl, but just didn't really do it more than about 3 steps. We were referred to a PT evaluation, and from there to the state program. They said she has weak core muscles and poor balance. We were advised a few things, such as not to put shoes or socks on her at all unless absolutely necessary. To get rid of anything like a walker (the kind you sit in, not the kind you push), an exersaucer, or a jumper, and that she should only ever spend 10 minutes at a time in one if we had to use one. They also gave us specific exercises to do with her and the therapist sees her once per week at her daycare. Now, at 17 months, she is able to stand unassisted for a few seconds, and the therapist says that she will take 2 steps in therapy, though I haven't ever seen her do it myself and we work on it every night.
My daughter is also delayed in other things besides motor. She is very delayed for communication as well. She doesn't say any words, or even very many different sounds. She also doesn't do things like point to ask for things. She does not follow simple directions (ie "get the book"). She doesn't really play with toys - she would much rather rip up paper and eat it! She tested low cognitively, but it's hard to tell at this point if that is due to the lack of communication. She receives therapy from the state for these issues as well. We don't know at this point if she is "just a late bloomer" as many people like to say, or if she has other issues going on. Her pediatrician has done some blood tests for certain conditions and all have come back as normal. Now we are referred to specialists for more evaluations and tests, but it takes a long time to get in so we are just waiting for our appointments. For now, she receives therapies and we figure that even if she's "just a late bloomer", it can't hurt to give her the therapies.
One thing that the docs have emphasized to me is that if she was only delayed in one area, then they would NOT be worried about it. So if she was not walking, but she was doing everything else just fine, or if she was not talking, but doing everything else, then they would not be doing this testing. It is only because she has "global" delays that we are being tested.