When using post-cache substitution

From ASP.Net 3.5 Unleashed (pg 1318)
When you use post-cache substitution (declaratively or programmatically) then caching no longer happens beyond the web server. Using post-cache substitution causes a Cache-Control:no-cache HTTP header to be included in the HTTP response, which disables caching on proxy servers and browsers. This limitation is understandable because the substitution content must be generated dynamically with each page request.