Blackberrys are seen as Personal Digital Assistants (PDAs); rather than mobile phones, because they combine the functions of a mobile phone with those associated with a computer. Therefore, they do not qualify for the normal mobile phone exemption for benefit purposes.
Provided that your sole motive in providing these PDAs, is to enable to the employees to perform their duties and they are prohibited from making significant personal calls under their contracts of employment, then there would not be an assessable benefit in kind.
In conclusion then, although Blackberrys are not mobile phones, they are unlikely to trigger a taxable benefit, provided that your employment policies make clear there is no private use.
Each case will be different, so if you would like advice in this area, please feel free to contact your local TaxAssist Accountant.