Rebooting won't cause any problems, but first try canceling the d/l then retrying. This usually works for me when the Market plays up. If you're still out of luck, try clearing the Market cache from Settings/Applications/Manage Applications, reboot and reattempt the installation.

If there is it's not national. Despite the storm we've experienced over the past 24hrs the Orange data network has been solid up this end of the country.