This is actually a nuget thing and not directly related to TeamCity. The official nuget feed behaves this way as well. To enable the behavior you desire do the following..
1) Download Nuget.exe Command Line Utility
2) Add Nuget.exe to your path, or store it in your Solution someplace
3) Add a prebuild event to your project
Nuget.exe update ..\..\Packages.config
4) Profit
1>------ Build started: Project: ConsoleApplication8, Configuration: Debug Any CPU ------
1>Build started 3/24/2014 3:00:00 PM.
1>PreBuildEvent:
1> C:\Nuget.exe update ..\..\Packages.config
1> Looking for installed packages in '..\..\..\packages'.
1> Updating 'ConsoleApplication8'...
1>GenerateTargetFrameworkMonikerAttribute:
According to the docs you can also target a .csproj file or a .sln (havent tried that though)