I'm ready for the 2010 run!
- Add a App_Code catalog to your web application, e.g. c:\inetpub\wwrwroot\wss\virtualdirectoris\80\App_Code
- Copy MySimpleService.asmx to c:\inetpub\wwrwroot\wss\virtualdirectoris\80 or to /Template/Layouts/..
- Copy MySimpleService.asmx.cs to c:\inetpub\wwrwroot\wss\virtualdirectoris\80\App_Code
- Try your service: http://sharepoint2010/MySimpleService.asmx
There are for certain issues, but it all depends on your scneario and what you need to do; KISS - Keep It Simple Stupid - but don't be stupid! :-) Of course you need to wrap-up and make a solution package of everything before you make it into a production environment...
Then the first thing to check is that the Build Platform target is set to x64.
If that doesn't do it for you, thenn hopefully Google will find the anser for you.
I've used both the solution and the step-by-step procedure with great success.