On 21st April 2014, Google made a change to their search algorithm that meant websites that aren't very well optimized for small touch screens will be less likely to appear in searches from mobile phones.
This means that if your website hasn't been specially optimized for mobiles, you may notice a drop in traffic and sales.
Of course, businesses will be affected differently. Only 7% of our traffic comes from mobiles and this makes sense intuitively since most people don't choose a web agency while out and about on their phone.
Some businesses may see over half their traffic coming from mobiles though and they should probably weigh up getting their site adapted to be "responsive" for smaller touch devices.
You can find out if your site is classed as "mobile-friendly" here: