In a recent post to his blog Evert Pot warns against some of the issues that mutual dependencies in your applications.

Much like most people, I try work out my class dependencies through a top-down 'waterfall'-ish approach. By attempting this, I think allows me to keep the structure very clear and understandable. [...] I try to apply the same model to instantiated objects and packages (groups of classes). When an object encapsulates another object, I attempt to make sure the sub-object object is not aware of the parent. When I design packages, I attempt to make sure 2 packages don't require 'each other'.

He gives an example of where this could cause problems - a Database logger that has three types of logging included: file, syslog and database. Obviously the last of the three requires the Database class so they must always be used/included together.

As a bonus a database-error could occur while logging, resulting in an endless loop (or segmentation fault if you're using PHP). [...] However, these types of situations are sometimes simply unavoidable (that's why we have include_once). When they are needed, they should be implemented with care and consideration.