Here are a few of the things you should take care of on your network / things you should know before you begin to Deploy or Migrate to ConfigMgr 2012:
1) Extending the AD Schema.
If this is a new deployment for you, or if you are migrating from SMS 2003, then you will want to seriously consider extending the AD Schema for ConfigMgr 2012. If you already have ConfigMgr 2007 deployed in your environment then you do not need to extend the schema again as the extensions are the same. Extending the Schema allows ConfigMgr to publish parts of its configuration to AD for the benefit of the SCCM Client. You MUST extend the schema if you plan on using Network Access Protection.
Here is a link to the TechNet documentation: http://technet.microsoft.com/en-us/library/gg682106.aspx
2) App-V Client
If you use App-V in your environment, you MUST use version 4.6 SP1 otherwise the ConfigMgr Client will not install.
3) Clean up your Collections / Software Packages
If you are using ConfigMgr 2007 then this information will apply to you. The migration process has a couple of limitations when it comes to Collections and Software Packages. On the collection side of things, if you have collections that have both users and comptuers as members they will not migrate. Also, if you have a collection query that is limited to searching inside a specific collection, the query will not come over. All your Software Packages must use a UNC path for its source directory.
4) SQL Here? Here? ... how about over Here ?
Placement of your SQL Database really depends on the size of your environment. If you manage a large number of devices, over 50'000, then you will be better served to use a stand-alone SQL server or SQL Cluster. If your environment is smaller than that, you can install it on the same server as ConfigMgr. Also consider that Secondary Sites will use SQL as well, though they can get by with having SQL Express installed on the local machine.
5) Site Codes
You cannot re-use side codes from your ConfigMgr 2007 environment, so plan for new Site Codes for your CAS, Primary, and Secondary Sites
More to come!