The biggest difference is the RAM and camera, other hardware spec. it is almost the same as P990. Of course, the user interface is different, to some, that makes all the difference, but there are also some who wants the P9xxx series interface.
Anyways, as for business phone, blackberry is more like an enterprise phone, companies give them out to their employees. Price does make a big difference, Blackberry is cheap compare to P1i.
SE never really have a phone for the enterprise users and P1i won't get them there. Yes, P1i is already cheaper than the Pxxx series (when they came out) but in the end, it is still for wealthy business users or executives who can afford to buy their own phones. Sort of like Nokia E90.
But for the people who can afford it, P1i is a good phone, could be cheaper, but then again, so can everything else.
P serise was never the enterprice smartphone from SE, and will never be one. SE has a great market plan on each individual serise, and design their phone accordingly. Unlike Nokia, SE has created a big difference in each serise as the product were created for different target market. P serise was never cheap and will never be cheap, due to the market segment. The only way to purchase a cheap P serise phone is to wait, wait till the coming of next P serise.
On the other hand, the blackberry is a cheap business phone, but not a smartphone. It was popular not by the price, but the "push mail" function. It only revealed the answer to "what does a business man really need from a cell phone?".
Compare P1i and P9xxx physically, you will only see the difference between size, camera and ram. However trust me that the new interface does mean something in P1i, or else why doesn't SE just keep the old one. SE's design team does not need to create anything further on the interface, just like Nokia. Anyway, they did which is to further enhance our on hand experience. Yes, it is true that different people get different feeling with the new interface, but at least they have tried to make it better. To be honest with you, I ain't a SE fan at all, and I use Nokia phone all the time (E61i is the best ever). But I have to say P1i is one hell good product.