Does anyone know why the $Action.Activity merge fields generate an error message in the Force.com IDE when used in S-Controls?

I have the following line in one of my S-Controls:
this.location.href="{!URLFOR( $Action.Activity.LogCall)}&what_id="+oppid+"&00N60000001QCYa=Outbound Sales Call&retURL=/"+hhid

This works fine if you save it through the Salesforce.com S-Control window, but if you do a "Save to Server" from the Force.com IDE you get the following error:
Save error: Field $Action.Activity.LogCall does not exist. Check spelling.  

It appears that all of the $Action.Activity merge fields display this same behavior.  Other merge fields from the $Action.Account and $Action.Asset groups appear to work fine.

Has anyone had any success in deploying CustomField changes to the User or Task standard objects using the Force.com IDE?

For the User object, I'm getting the following error message:
  unable to obtain exclusive access to this record

For the Task object, I'm getting the following error messages:
  Field 'Subject' in 'Task' is not a picklist field
  Entity Enumeration Or ID: bad value for restricted picklist field: Task

I have been able to deploy changes on the Account and Opportunity, but have been unsuccessful in deploying to User and Task objects.

Thanks

Does anyone know why the $Action.Activity merge fields generate an error message in the Force.com IDE when used in S-Controls?

I have the following line in one of my S-Controls:
this.location.href="{!URLFOR( $Action.Activity.LogCall)}&what_id="+oppid+"&00N60000001QCYa=Outbound Sales Call&retURL=/"+hhid

This works fine if you save it through the Salesforce.com S-Control window, but if you do a "Save to Server" from the Force.com IDE you get the following error:
Save error: Field $Action.Activity.LogCall does not exist. Check spelling.  

It appears that all of the $Action.Activity merge fields display this same behavior.  Other merge fields from the $Action.Account and $Action.Asset groups appear to work fine.

Has anyone had any success in deploying CustomField changes to the User or Task standard objects using the Force.com IDE?

For the User object, I'm getting the following error message:
  unable to obtain exclusive access to this record

For the Task object, I'm getting the following error messages:
  Field 'Subject' in 'Task' is not a picklist field
  Entity Enumeration Or ID: bad value for restricted picklist field: Task

I have been able to deploy changes on the Account and Opportunity, but have been unsuccessful in deploying to User and Task objects.

Thanks