I have been asked this question a few times this week, so I thought I should find out and then of course blog the answer.  The SoftGrid Product team blogged the following, which outlines the key differences.

SoftGrid 4.1 Service Pack 1

SoftGrid 4.1 SP1 is mostly a hotfix and critical updates release, improving overall system stability and compatibility. A noteworthy improvement is the ability to sequence applications using Side by Side assemblies (aka WinSxS), including Visual Studio Express and Office 2007, among many other applications from both third party ISV's and Microsoft. Expected availability is in the second quarter of this year and the release will be made available for download as soon as test is completed. We will release the Sequencer, Desktop Client, Terminal Server Client and Server at the same time. 

SoftGrid 4.2

SoftGrid 4.2 builds on the 4.1 improvements by adding Windows Vista compatibility, both on the sequencer and client.  There is not going to be a 4.2 version of the Server or the Terminal Server client as the moniker 4.2 is reserved for the components adding Windows Vista compatibility. We expect a final release some time around Q3 of this year. We expect 4.1 SP1 and 4.2 to maintain parity over time, the only exception being Windows Vista compatibility in the 4.2 components.