Do Chrome extensions access iframes?

Content scripts defined in the manifest (with "all_frames": true) will run on newly created iframes. What matters is that a new navigation starts for every frame, and content scripts are scheduled to be injected at that point.

In contrast, if you dynamically inject code with chrome.tabs.executeScript(), then it will only be injected in the frames present at the time you call it. You'd need some mechanism to detect new frames (Mutation observers? webNavigation API?) if you want to keep up with them.

Yes, a Chrome Extension "content script" can run in all iframes (that are initially in the HTML when the page is loaded). In order to have the content script run in all frames you need to configure it to do so in the Chrome Extension manifest.json using the all_frames property:

http://code.google.com/chrome/extensions/content_scripts.html

{
  "name": "My extension",
  ...
  "content_scripts": [
    {
      "matches": ["http://www.google.com/*"],
      "css": ["mystyles.css"],
      "js": ["jquery.js", "myscript.js"],
      "all_frames": true
    }
  ],
  ...
}

No, the content scripts will NOT execute in the iframes loaded dynamically via JavaScript in the page.