mobiEnthusiast.mobi has an article on the essential must-have features for a mobile website.

1. Your company name and logo.
2. What products or services your company provides.
3. How can people buy from you? Click to call is a great feature for this, especially since mobile shopping carts are hard to find.
4. How can customers contact you? You can include an email form or click to call feature on your site.

 It would be interesting to see a site with just these pieces on, to see whether it is enough or not.

"What's the least I can put on my mobile site?" It comes down to what kind of experience your customers are looking for, in general?

There will be customers who really just want to get onto your site and buy the product they know they want. But how many customers would look at a 'starting run' bare-bones site, not like the experience (or lack there of) and then never return to it - even when it is completed?

Is it better in the long run to have a 'head start on your competition', or is it better to wait and release a fully competent site when it's ready? Does waiting until you can release a full featured site give a better impression to your customers?

Biomehanika:

But how many customers would look at a 'starting run' bare-bones site, not like the experience (or lack there of) and then never return to it - even when it is completed?

A very valid point.  First impressions are important and if you blow it, they will never return.  Bare bones ... to me it has to be a complete experience the first time a visitor comes.

 Just my thoughts....

As a mobile web designer I am constantly trying to tweak things to get them to work on the widest range of devices possible. I think what is more important than even those four things lies in the experience the person has. They need to be able to have a great design that grabs their attention but maintains an easily navigable interface.

I thought it was cool how they brought up the click to call feature. As a designer, I find this extremely beneficial in building contact information.