Some friends here on the Hyper-V team shared a PowerShell 2.0 script for creating a dynamic VHD for your VM:

# Create a dynamic VHD

param(
    [string]$vhdPath = $(throw "Must specify full path for new VHD"),
    [string]$vhdSize = $(throw "Must specify size for new VHD (in GB)")
)

# Size in bytes
$GB = [System.UInt64] $vhdSize*1024*1024*1024

#obtain the Msvm_ImageManagementService class
$ImageMgtService = get-wmiobject -class "Msvm_ImageManagementService" -namespace "root\virtualization"

# Create the Dynamic VHD
$result = $ImageMgtService.CreateDynamicVirtualHardDisk($vhdPath,$GB)

if($result.ReturnValue -eq 4096){
    # A Job was started, and can be tracked using its Msvm_Job instance
    $job = [wmi]$result.Job
    # Wait for job to finish
    while($job.jobstate -lt 7){$job.get()}
    # Return the Job's error code
    return $job.ErrorCode
}
# Otherwise, the method completed
return $result.ReturnValue

For more info on how to use PS cmdlets see: http://www.microsoft.com/technet/scriptcenter/topics/msh/cmdlets/index.mspx

See also James O’Neil’s New and improved PowerShell Library for Hyper-V. Now with more functions and... documentation!

For all 35 sample Hyper-V PS1 scripts in a zipfile, go to: Hyper-V PowerShell Example Scripts.zip-download