Is there a tool like Furigana that I can use on iOS Safari? On my iPod (iOS 4.1), I'm often browsing Japanese webpages, and while I can usually understand the meaning, I often don't know the readings, especially in new contexts.

For my desktop web browsers, there are various ways to get it to display furigana for the webpage you're looking at: the Furigana Inserter extension, a bookmarklet to hiragana.jp, etc.

Does iOS Safari support extensions, or bookmarklets, or any other mechanism by which I can have it add furigana?  I can manually copy the URL, and go to hiragana.jp and paste it, and it looks great when it's all done, but the process is manual and slow.  Alternatively, is there any built-in furigana support, for Safari or any other app?  (If there is it's probably well-hidden, but it wouldn't be the first well-hidden part of iOS!)

I'm not too concerned at this point about whether it's built-in functionality, or whether it has to send my webpage unencrypted over the internet to a service.  It would be really nice if it was local-only and secure, but at this point I'll take anything.
You can use a bookmarklet. Just sync your Safari or Firefox bookmarks with iTunes. Every bookmarklet you have on the desktop will be available on mobile Safari on the iPhone.

Or you can add the bookmarklet directly from within mobile Safari on the iPhone:

Step 1: Select the line bellow and copy it:

javascript:void(document.location='http://trans.hiragana.jp/ruby/'+escape(document.location))


Step 2: Navigate to hiragana.jp and tap on the '+' button to save a bookmark

Step 3: tap on the bookmarks icon then on 'Edit'

Step 4: Select the bookmark you just added

Step 5: tap on the address, then on the gray 'x' to clear the field

Step 6: tap that field again and select 'Paste'

Step 7: you're done