There were several good suggestions and great discusion about the business case for Groupon. However, we did not get around to summarizing what technical changes should be done to your Joomla site to prepare for the wave of Groupon traffic.
I thought I would put some of my thoughts together from the evening.
1. Create a lightweight Landing Page.
If you have the technical ability, you may want to turn this page into an html flat file. By doing this you completely remove the page from your Joomla structure. An html flat file will load much quick and not put any added stress on your mysql datase.
2. Turn on a Cache
Having a cache turned on will reduce quite a bit of stress for the server. You can enable the Joomla Cache, look for a good 3rd Party Joomla Cache extension or go with a good serverside cache such as mem cache or varnish. Here is a good article about Using Jooma and Varnish together. Keep in mind sufficient testing should be done to ensure that the cache is not breaking any of your pages.
3. Inform your Hosting Company
Let you hosting company know what is coming. They may be able to move your site to a better server or remove any cpu or bandwith restrictions. You do now to have your account temporaliy disabled because of an overage. Keep your hosting company in the loop. Who knows they may even have some suggestion you have not though about. If you are with one of the larger hosting companies I am sure have had sites get hit by Groupon Before.
5. Use a Content Delivery Network (CDN)
A good content delivery network will really speed up the site. True content delivery networks can be expensive but there are more and more low cost options for CDNs try looking at Cachefly or MaxCDN. MaxCDN even has a joomla plugin to make the process even easier.
Hopefully you found some of the suggestions helpful. We just upgraded this site and set up a new blog extension. We hope to have the commenting system worked out soon.