When you've all stopped frothing at the mouth....
It could actually be a good idea....BUT...a few things need to be thought about first.
1. Secure Server for transmitting and receiving emails and data
2. Locked down policies to prevent unauthorised apps and data usage
3. Password protection policies and remote wiping set-up
You have to remember that enterprise businesses have been have been handing out expensive BlackBerry's for years and when I had mine at work you couldn't even use the camera due to the policies and only certain people could use the browser. The government would be far better talking to Apple directly who would no doubt give them a licensing deal and help setup the back-end services that would be needed to make it a workable solution. The MP's would all need training too on how to keep the device secure and how to use the applications they need.
I would suggest that they would be unable to use the App store at all but bearing in mind how locked down Apple's AppStore is at least there is less chance of rougue applications stealing MP's (and our!) details. I would be terrified if they were let loose on Android pads! I still don't see why they should have the top of the range devices but I'm pretty sure they would get either a bulk buy deal for considerably less than full price or they would get a deal through 3 or Vodafone.
It sticks in the craw because we have this image of MP's as caniving scum stealing all our money but in the vast majority of cases that's just not true. Take away 1 of their laptops, give them an ipad and cover with a built in BlueTooth keyboard. Job done.