I've got a few forms on my web app that require a start date, which is always the first of a month. Currently i'm using the jQuery date picker plugin that disables every day of the month except the first.
you can see how they work here: http://jqueryui.com/demos/datepicker/
it's a little bit confusing to our end users to see all the days of the month when they can only pick the first. But it keeps the interface more consistent with other places where dates are picked.
I can see 2 other options for these interfaces.
Using a year and month drop down that have reasonable options for the year, and the 12 months for the months.
Have the date picker, and allow them to pick any date, and just switch it to the first.
Keep it the same as it is now, with all the days except the 1st disabled.