If your Web part is simply displayed in a SharePoint page, without using the SP API, you could simply host it in an ASP.NET page on your laptop, but this is not a common scenario.
One new feature of SP 2010 is the client API, i.e. a subset of the full API that can be used outside of the farm. If your usage of the API fits in this subset, this could be useful, but you will still need to access a SP server somewhere.
Another option is to put all the code that uses the SP API in something similar to a Database Access Layer which talks to SP on one hand and returns business objects (not lists or lists items). This way, you could simulate this part on your laptop and concentrate on the look of the Web part and its business rules, without SP. If this part is in its own DLL, the only reference to SP DLLs would be there, so the project on your laptop would not need to reference the SP DLLs.