When you use Google Search from your mobile phone, you see two kinds of web pages results: traditional web pages (the same ones that you’ll see from your desktop) and mobile web pages.
To build its index of mobile web sites, Google uses a different bot than the traditional GoogleBot. The bot, named Googlebot-Mobile, actually crawls any website it finds and if it is considered as mobile friendly, it will then be included in Google mobile web index.
So if your mobile site is linked from other sites (be it mobile sites or regular web sites), Googlebot-Mobile has probably already found it and it is included in its mobile web index.
If that’s not the case, you can create a Mobile Sitemap of your site and submit it via Google Webmaster Tools.
Now, regarding the crawl frequency of Google Mobile Bot, it is of course less frequent than the regular one. While Googlebot usually comes to a website everyday, you can sometimes see Googlebot-Mobile not coming to back to your site before a period of 2 weeks.
The chart below is an example of the crawl rate made by Googlebot-Mobile on of the mobile sites that I manage over the last 4 months. As you can see, Googlebot-Mobile came more often in the recent months, but I don’t know if it’s a natural increase on their side or due to the site having more links (that’s the only factor that changed).
Click to enlarge:
Mobile site owners should be aware of Googlebot-Mobile’s user agent. In fact, it behaves as a mobile phone, because sometimes only accesses made via a mobile phone are allowed to a site (for example, you sometimes can’t log on to a mobile web site from your PC) Google has chosen to identify its mobile bot as a cell phone.
Googlebot-Mobile’s full user agent is currently this one:
Nokia6820/2.0 (4.83) Profile/MIDP-1.0 Configuration/CLDC-1.0 (compatible; Googlebot-Mobile/2.1; +http://www.google.com/bot.html)
As you can see, it behaves as a Nokia 6820, so you will need to remove this user agent in your mobile visits report to make sure you only count those made by real mobile phone users.