My DS takes piano and has for about 3 1/2 years. We've always had a piano (I inherited an antique upright). He has to practice on a keyboard at his grandparent's house sometimes and it's definitely a very different experience and practice than he gets on the "real" piano. But I don't know if it would have made that big of difference the first year or even 2 (he started at 5). His teacher is working on things like finger strike on really long, fast runs. Our teacher required a regular piano on day one (piano snob!). DS does love to play around with the keyboard when he gets the chance though! I actually do wish we had one for travel, if nothing else.
I think the suggestion to keep your eyes peeled for a used piano is a really good one if you'd like to have one in your home eventually. I have several friends that had to give pianos away to get them out of their homes.
