In a Google Groups thread, AdWordsPro Sarah offers some nice optimization tips for mobile users. She acknowledges that they shouldn't be ignored; there are 405 million mobile users worldwide, she says.
If you want a business presence in the mobile realm, you can create a business page for mobile ads.
Additionally, for your Google AdWords campaign, make the following considerations: your website should be mobile friendly (or have a mobile friendly website available for mobile users only). Your keywords should be 1-3 words, broad match, with general terms. You can use geo-modifiers for local businesses.
The ad text should be short and compelling (18 characters) and the text itself should also be 18 characters. You can use text speak (e.g. CUL8R).
Google also has a mobile FAQ for more insights.
Forum discussion continues at Google Groups.