I have never used their "mobile" site. I've always used PIE to do online banking straight from my Treo with Citibank. Just go to this site:
https://web.da-us.citibank.com
If you are sent to a page where it will tell you your browser/OS will not work - just scroll down and hit the "Continue" button. At some point when the next page finishes loading it may prompt you to accept the certificate, click "Yes". Then scroll almost all the way down and you'll see a user name and password box. Just type in your Citi Online banking user name and password and it will then nicely display a slew of info including your last visit, and all your account summaries. And from there you can click on all of your accounts for more info, and do payments/transfers, etc...
While it's probably not a swift as their "mobile" site it still works fine for me...
Verizon 700w