The "mainframe" will be around for a long time. It may not run on the current hardware, but there is such an investment in the application technology that it cannot be re-written, re-placed quickly.
More and more smaller applications (especially departmentsl systems) are being implemented on non-mainframe platforms. Thereis still much working being done on the mainframe. Keep in mind that once implemented, the system will probably be used for several years - which as much work (or more) will be spent on maintainng the sstem or adding new features (whether on Windows, *NIX, or the manframe.
Yup, COBOL developers will be needed for years.
Hope this helps,