Not everyone has an iPhone and I'm sure this would be a welcome feature in the mobile Google Reader. Fortunately, the feature already exists, even if not in Google Reader, but in the mobile Google Search. To go from Google Reader to Google's homepage, click on "More Google Products" and select Search from the list. If you search the web using Google and click on a search result from a site that has feeds, the feeds are listed at the top. Clicking on a feed, you'll be able to preview it in Google Reader and also subscribe to it.
If you already know the address of a feed, you can use this URL to subscribe to it:
(replace <FEED_URL> with the actual URL)