I happily wrote a blog post on this a little while ago... http://www.projecttoomanycooks.co.uk/blog/?p=1351 :) It's got screenshots and everything... (I've put the text, without the screenshots, below)
We're going to enable an extra keyboard on your iphone, and show you how to use it to get the symbols out.
Step 1: Switch on the keyboard
Nagivate to Settings->General->International->Keyboards->Add New Keyboard and choose 'Japanese, Ten Key'. Once it's selected, your Keyboards page should look a little like this:
Step 2: How to access and ^_^ symbols
Now open up a typing screen (we're using SMS in this example). You'll notice a globe has appeared between the '123' button and the spacebar, this button switches between international keyboards.
Make sure that you are in the text window (rather than the number window shown in the screenshot, and press our new globe button. You'll find yourself on a japansese page (press it again to go back). Should look like the screenshot below
Of interest here is the '^_^' button on the bottom row. That gives you access to a massive range of smilies - it's not the main point of this tip, but it's pretty handy... two photos below
Step 3: Actually putting the icons in
So now the final part - from the Japanses ten key keyboard, press the 'ABC' key twice. You should find yourself on a numbers page. Looks like this:
You're now going to press and hold the '1' key. You'll see a star appear to the left: slide over and press it, if you want a star, you're done. But if you want more symbols, press the arrow at the end for more of a range.
There's lots more symbols around that area, feel free to have a play. Have fun!