It is not possible to pin SQuirreL to the taskbar on windows 7. This is apparently because it runs under the javaw.exe executable. Any program with this for a process name cannot be pinned. This is likely because it is an ambiguous process name and could be confused for any other amateur java application.
While I hate to suggest solutions in a bug report (often this can obscure a more elegant solution the developer would have otherwise thought of), but a possible solution would be to create a SQuirreL.exe file to launch the client, rather than using a batch script to call javaw.