Result table (SunSpider 0.9.1, all devices running iOS 5)

<td>
  iPhone 4S
</td>

<td>
  iPhone 4
</td>

<td>
  iPhone 3GS
</td>
<td class="positive">
  2270ms
</td>

<td>
  3483ms
</td>

<td>
  4903ms
</td>
<td class="positive">
  281ms
</td>

<td>
  486ms
</td>

<td>
  672ms
</td>
<td class="positive">
  279ms
</td></td> 

<td>
  410ms
</td>

<td>
  614ms
</td>
<td class="positive">
  177ms
</td>

<td>
  200ms
</td>

<td>
  270ms
</td>
<td class="positive">
  20ms
</td>

<td>
  26ms
</td></td> 

<td>
  34ms
</td>
<td class="positive">
  164ms
</td>

<td>
  211ms
</td>

<td>
  315ms
</td>
<td class="positive">
  332ms
</td>

<td>
  528ms
</td>

<td>
  770ms
</td>
<td class="positive">
  219ms
</td>

<td>
  433ms
</td>

<td>
  553ms
</td>
<td class="positive">
  88ms
</td>

<td>
  117ms
</td>

<td>
  151ms
</td>
<td class="positive">
  711ms
</td>

<td>
  1072ms
</td>

<td>
  1525ms
</td>
Test
Total
3D
Access
Bitops
Controlflow
Crypto
Date
Math
Regexp
String

Raw results

iPhone 4S (iOS5)

iPhone 4 (iOS5)

iPhone 3GS (iOS5)

Bonus: FishIETank (10 fish) (Canvas test)

<td>
  iPhone 4S
</td>

<td>
  iPhone 4
</td>

<td>
  iPhone 3GS
</td>
<td class="positive">
  50fps
</td>

<td>
  35fps
</td>

<td>
  25fps
</td>
Test
Total

One thing to note here: the 3GS has a bit of an advantage because of a non-retina screen. However, it's still outperformed easily by the iPhone 4 and 4S.

JavaScript SunSpider benchmark: iOS 4.3 vs iOS 4.0
BlackBerry Torch SunSpider results (JavaScript benchmark)
iPhone 4 SunSpider test results (22% faster than iPhone 3GS)
JavaScript SunSpider test: iOS 3.1.3 versus iOS 4 GM
JavaScript SunSpider: HTC Evo versus HTC Incredible versus Nexus One