It's possible to use HTML5 input types to hint to the OS as to which keyboard should be displayed.
There are numerous webpages out there that list all the new HTML5 input types and show examples of how they look on different platforms.
Windows Phone 7, which uses IE9, does not support these newer HTML5 input types.
Windows Phone 8 and Windows 8 do as they use IE10.
For a full list of feature support, there's a comparison chart from quirksmode.org and another on wufoo.com