From the description in the Windows Phone Store:
Bitcoin Rate Lookup allows you to view the current bitcoin exchange rate for most world currencies. Quickly change the currency you want to check the exchange rate for. See both the exchange from bitcoin to your currency or your currency to bitcoin.
Bitcoin Rate Lookup also allows you to pin an active tile to your start screen for quick viewing of the current exchange rate for your currency without having to load the app.
Uses the Coinbase exchange for current rate lookup.
As with everything else that uses the live tile system on Windows Phone, there are natural restrictions imposed by Windows Phone itself, according to battery and connectivity status. The upshot of which is that you can be fairly sure that the rate quoted on the tile is within about 20 minutes of the current value - the exact time the background agent gets run and live tile updated are uncertain. I've seen it as short as 10 minutes and as long as 30 - it's your call as to whether this is up to date enough for your Bitcoin interest!
Here's a quick walkthrough of this simple but useful utility:
The app itself updates on-demand, of course, though only from one particular Bitcoin exchange - there are half a dozen equally reputable exchanges around the world (not Mt Gox now, of course!) and it would have been nice to have had the choice; (right) picking a currency to compare Bitcoin against.
There are three live tile sizes supported, though the smallest is just an icon so hasn't been shown here. Of the other two, both seem to underuse the space available. I'd have liked to have seen the large one include either more data or a pretty graphic; while the medium tile could definitely do with a timestamp....
You can grab Bitcoin Rate Lookup here in the Store - it's a free download, so comments welcome - is a half an hour delay on the value a problem for you?