It works for me.
I found an email in my inbox on my phone, picked a sentence and typed it in spotlight and a few words in, it brought up the email. Am I understanding you correctly? It may be because you're on an iPod Touch and away from WIFI so the iPhone cant access your email offline.
I just tried it again, and I have good Wi-Fi and it's not working. I Googled and from what I found said you could search to, from, and subject but not content. Something to do with indexing. I just wanted to check the forum to see what's going on.
But I can only assume, since there is the option to search "all" alongside "from" "to" and "subject," there will be a firmware update in the future which will allow searching the body of messages.