Question: To interact with an external data feed I need to pass a rolling security key which has been MD5 hashed (every day we need to generate a new MD5 hashed key).

I'm trading up whether or not to do it every time we call the external feed or not. I need to has a string of about 10 characters for the feed.

It's for an ASP.NET (C#/ .NET 3.5) site and the feed is used on pretty much every page. Would I best off generating the hash once a day and then storing it in the application cache, and taking the memory hit, or generating it on each request?

Answer: 
Generate some sample data. Well, a lot of it. Compute the MD5 of the sample data. Measure the time it takes. Decide for yourself.
Using the Asp.Net cache is very easy so I don't see why you shouldn't cache the key. 

Storing the key in cache may even save some memory since you can reuse it instead of creating a new one for each request.