My site has quite a deep navigation structure and quite often it looks like the out of the box navigation is leaking memory, especially the SPWeb objects.

The log message looks like

Potentially excessive number of SPRequest objects (14) currently unreleased on thread 5. Ensure that this object or its parent (such as an SPWeb or SPSite) is being properly disposed.

+4  A: 

Stefan Goßner's blog post seems to answer the question. The issue is not that the SPWeb objects are not being closed, but that once a certain threshold (defaults to 8) of allocations are hit, the warning is created in the log. Depending on your site structure the number that will be created will vary.