Simplified way to get assembly description in C#?

I would use an extension method for ICustomAttributeProvider to provide a strongly typed GetCustomAttributes which returns a strongly typed enumerable. The only linq usage would be the call to FirstOrDefault and OfType

public static void Main() {
    Assembly assembly = Assembly.GetExecutingAssembly();
    var descriptionAttribute = assembly
        .GetCustomAttributes<AssemblyDescriptionAttribute>(inherit: false)
        .FirstOrDefault();

    if (descriptionAttribute != null) {
        Console.WriteLine(descriptionAttribute.Description);
    }

    Console.ReadKey();
}

public static IEnumerable<T> GetCustomAttributes<T>(this ICustomAttributeProvider provider, bool inherit) where T : Attribute {
    return provider.GetCustomAttributes(typeof(T), inherit).OfType<T>();
}

There isn't, really. You can make it a bit 'more fluent' like this:

 var descriptionAttribute = assembly
         .GetCustomAttributes(typeof(AssemblyDescriptionAttribute), false)
         .OfType<AssemblyDescriptionAttribute>()
         .FirstOrDefault();

 if (descriptionAttribute != null) 
     Console.WriteLine(descriptionAttribute.Description);

[EDIT changed Assembly to ICustomAttributeProvider, cf. answer by Simon Svensson)

And if you need this kind of code a lot, make an extension method on ICustomAttributeProvider:

 public static T GetAttribute<T>(this ICustomAttributeProvider assembly, bool inherit = false) 
 where T : Attribute 
 {
     return assembly
         .GetCustomAttributes(typeof(T), inherit)
         .OfType<T>()
         .FirstOrDefault();
}

Since .Net 4.5, as Yuriy explained, an extension method is available in the framework:

var descriptionAttribute = 
    assembly.GetCustomAttribute<AssemblyDescriptionAttribute>();

var attribute = Assembly.GetExecutingAssembly()
                    .GetCustomAttributes(typeof(AssemblyDescriptionAttribute), false)
                    .Cast<AssemblyDescriptionAttribute>().FirstOrDefault();
if (attribute != null)
{
    Console.WriteLine(attribute.Description);
}