Patrick Smacchia wrote an excellent post on how you should partition your code in multiple assemblies. Because this is one of those subjects that i'm pretty opinionated about, i figured i'd quickly recap the valid and invalid reasons for creating assemblies that Patrick highlighted.
Valid reasons for creating a new assembly:
Tier separation: separating the code you will run in different processes. For instance: having an assembly for client-side only stuff, and one for server-side only stuff
Avoid premature loading of large pieces of code that aren't always necessary: if you put the 'optional' code in a separate assembly, that assembly won't be loaded until you actually need it.
Framework feature separation: if your framework offers types that will never be used together (for instance: types for web development vs types for windows development) then it doesn't really make sense to put them in the same assembly.
AddIn/Plugin model: there are many valid reasons for putting plugins in their own separate assemblies
Tests: i personally don't like to put my test code next to my production code, so i always put that in a separate assembly
Shared types: all 'common' types that you would like to use in different tiers
Invalid reasons for creating a new assembly:
Assemblies as units of development: there's not a single source control management system that doesn't make it easy for multiple people to work on the same assembly
Assemblies as units of reusability: this one wasn't on Patrick's list, but it is a pet peeve of mine. I've often seen people separate things into multiple assemblies so each part could be reused separately. Which is a prime example of Intellectual Masturbation Syndrome if there's no actual need to reuse each assembly separately.
Automatic dependency cycle detection: when assemblies have cyclic dependencies on each other, Visual Studio automatically notifies you of the problem. Patrick obviously recommends using NDepend to prevent such problems. I'd recommend thinking before/during/after you write the code
Using internal visibility to hide implementation details: if you want to hide implementation details you can use the internal access modifier so the 'internal' parts are only available to the types contained in the owning assembly. I seriously dislike this approach. Using the internal access modifier is often a good sign that there is something wrong with the design. Hiding implementation details is not always the same thing as 'proper encapsulation'!
Using internal visibility to prevent usage from the rest of the code: Yet another approach that i seriously dislike. I prefer to use the Published vs Public approach