Does swift playground support UIKit?

YES, it does!

File: New > File... > iOS > Source > Playground

import UIKit
let lbl = UILabel(frame: CGRect(x: 0, y: 0, width: 300, height: 100))
lbl.text = "Hello StackOverflow!"

Then, save the file. (Or manually run it.) This will trigger the Playground to interpret UI related things. At this point, the word "UILabel" should appear on the right-hand side.

Now, to actually view what you've done, you've got to click on the "Quick View" eye on the right, or the white circle to open it in Assistant Editor:

Here's a screenshot of some basic things with UIImage working, etc.

(EDIT: minor text update to current CGRect syntax -- But, screenshots still show old syntax.)

In Xcode 7, now you can't use the Quick Look to see the appearance of a UIView.

Instead, use the Assistant Editor and:

XCPlaygroundPage.currentPage.liveView = sampleView

Like this:

import XCPlayground
import UIKit

XCPlaygroundPage.currentPage.needsIndefiniteExecution = true

// Simulate User Interaction, not available in Xcode 7.2
func delay(delay:Double, closure:()->()) {
    dispatch_after(
        dispatch_time(
            DISPATCH_TIME_NOW,
            Int64(delay * Double(NSEC_PER_SEC))
        ),
        dispatch_get_main_queue(), closure)
}

let color = UIColor(red: 1, green: 1, blue: 0, alpha: 1)
let leftMargin = 20
let view = UIView(frame: CGRect(x: 0, y: 0, width: 375, height: 667)) // iPhone 6 proportions
view.backgroundColor = UIColor.grayColor()

// LABEL
let label = UILabel(frame: CGRect(x: leftMargin, y: 5, width: 300, height: 44))
label.text = "Hello, playground"
label.textColor = UIColor.whiteColor()
view.addSubview(label)

// TEXTFIELD
let textField = UITextField(frame: CGRect(x: leftMargin, y: 60, width: 300, height: 44))
textField.placeholder = "Edit me…"
textField.backgroundColor = UIColor(white: 1, alpha: 0.5)
textField.textColor = UIColor.whiteColor()
textField.userInteractionEnabled = true
view.addSubview(textField)

XCPlaygroundPage.currentPage.liveView = view

delay(1.0) { () -> () in
    textField.text = "New text!"
}

Edited@2014-11-13: It seems the new xcode 6 had fixed this.

NO, It doesn't. But it's worth noting that you can import UIKit.

If you want to import UIKit you cound follow this:

1. View -> Utilities -> Show File Inspector (opt + cmd + 1)
2. On the right side of Xcode Change “Playground Settings -> Platform” from OS X to iOS

then you could import UIKit or some module for iOS

ps. I try to create a UIImageView but it doesn't show the correct image on the right side. It seem worthless to import UIKit