I have build a webpart and imported it into SPS v2, and it works
just fine.

But when I try to use the SPS ObjectModel, I get this error:

Request for the permission of type
Microsoft.SharePoint.Security.SharePointPermission,
Microsoft.SharePoint.Security, Version=11.0.0.0, Culture=neutral,
PublicKeyToken=71e9bce111e9429c failed.

Here's a sample of my code:

using Microsoft.SharePoint;
...
SPWeb web = SPControl.GetContextWeb(Context);
SPList list = web.Lists[0]; //The exception is thrown here
output.Write("List name : "+list.Title);

What is the problem?