If I am unable to contribute to the Roth IRA do I just hold onto it?

Yes, you just hold onto it and let it grow. In the years that you can contribute, I would recommend doing so. You never know how things will change relative to your income and even the tax laws, so having that money invested and growing and adding as you are allowed is a reasonable plan.

We are in a similar situation where we have only been able to contribute to a Roth IRA twice, but there is a chance that we will be below the income limits for 2007, and if we are, then I will make a 3rd contribution into both DH's and my Roth. Since my DH is self-employed, his income fluctuates wildly, so I just wait until I do our taxes, and if we are elibible, that's when I make the contribution.

You may find a similar strategy works in your situation as well.