Here is the script (SCOM 2007 maintenance mode):

param($rootMS,$minutesMM,$comment,$textFile) 
$servername = Get-Content $textFile 
Add-PSSnapin "Microsoft.EnterpriseManagement.OperationsManager.Client" -ErrorVariable errSnapin
Set-Location "OperationsManagerMonitoring::" -ErrorVariable errSnapin
new-managementGroupConnection -ConnectionString:$rootMS -ErrorVariable errSnapin
set-location $rootMS -ErrorVariable errSnapin
foreach ($srv in $servername) { 
  $computerPrincipalName = $agent.displayname 
  $computerPrincipalName 
  $computerClass = get-monitoringclass -name:Microsoft.Windows.Computer 
  $computerCriteria = "PrincipalName='" + $computerPrincipalName + "'" 
  $Computer = Get-MonitoringObject -monitoringclass:$ComputerClass | where {$_.PathName -match $srv} 
  $startTime = [System.DateTime]::Now 
  $endTime = $startTime.AddMinutes($minutesMM) 
  "Putting " + $computer + " into maintenance mode" 
  New-MaintenanceWindow -startTime:$startTime -endTime:$endTime -monitoringObject:$computer -comment:$comment 
}

It reads a text file and puts each server from that text file into maintenance mode. It does each server one at a time and only goes to the next one after the previous one has finished. You can imagine this can take a while with a lot of servers. Does anyone know a way to speed it up?

I'll have to look a bit closer for some perfomance suggestions (should have some time this weekend), but you might have some unintended consequences with your "-ErrorVariable" assignments.
By default, the ErrorVariable will replace the existing value with the new error.. that means if you have an error in Add-PsSnapin and one in Set-Location, the Set-Location error will overwrite the Add-PSSnapin error.
If you add a "+" in front of the variable name, it will add the new error records, rather than replacing them.
E.g. -ErrorVariable +errSnapin

Everything that's somewhat static, move it before the foreach.

Example:
$Computer = Get-MonitoringObject -monitoringclass:$ComputerClass

Move that to be eval'd before the foreach, then use $Computer within your foreach.

Move all your date calculations before the foreach also since you don't need to calculate them each time.

If you're still stuck, let us know. I might be able to provide more feedback on Monday or Tuesday...

If you have PowerShell v2.0 you can use a Start-Job command. If you put it inside the foreach loop, the servers will process in parallel, rather than in series.