Trunk Notes is a personal wiki for the iPod Touch. See here and here . It can be bought for $3.99, if you have an iTunes account.
It uses MarkDown for formatting. One thing I don't like about it is that it uses only WikiWords (i.e. CamelBack spelling, or two capital letters in a word, or in their lingo: "A WikiWord is two or more words squashed together with each word beginning with a capital letter"). The other is that I don't use an iPod, even though I have been tempted more than once to buy one. :)
Given the recent move by apple to restrict the iPod even more by forcing a software developer to remove USB support from its Stanza reader, I am inclined to resist temptation even more. Why would I want a device that is ever more tightly controlled by a company that tells its "customers" how they can and cannot use a device I have paid good money for? The very concept of a device that needs "jailbreaking" to make it freely usable seems very Orwellian to me.
1. See also MyWiki, which needs an iPhone (by which I was never even tempted :)).
2. Even more unsettling is that the developers could not discuss the reasons for having to remove USB support because they were strictly "forbidden from discussing the specifics of [their] conversations with Apple on this matter".