Now with support for SuSE Linux Enterprise Server 10! I see that version 2.0 of the VM Additions for Linux have been uploaded, and you can now get them directly from the MS Download site instead of Connect.  While they are designed for Virtual Server 2005 R2 SP1, they should probably work in Virtual PC.  Your mileage may vary... try at your own risk.

From the download site:

Virtual Machine Additions information
To improve the interoperability between virtual machines running Linux guest operating systems and Virtual Server 2005 R2, Virtual Machine Additions are available for Linux guest operating systems. You can install these Virtual Machine Additions components in your Linux guest operating systems to improve the following operating system capabilities:

• Guest and host synchronization for time synchronization, for heartbeat generation, and for coordinated shutdown operations
• Mouse driver
• Display driver
• SCSI hard disk emulation

Note: The SCSI hard disk Virtual Machine Additions component requires Virtual Server 2005 R2 Service Pack 1 (SP1).
Qualified Linux guests:
Enterprise distributions

• Red Hat Enterprise Linux 2.1 (update 6)
• Red Hat Enterprise Linux 3 (update 6)
• Red Hat Enterprise Linux 4
• SuSE Linux Enterprise Server 9
• SuSE Linux Enterprise Server 10

Standard distributions

• Red Hat Linux 7.3
• Red Hat Linux 9.0
• SuSE Linux 9.2
• SuSE Linux 9.3
• SuSE Linux 10.0

Download them here: http://www.microsoft.com/downloads/details.aspx?familyid=bf12642f-77dc-4d45-ae4e-e1b05e0a2674&displaylang=en&tm