I don't know of any way someone could attack the data on the Blackberry via Wi-Fi. As long as the Wi-Fi router is using strong encryption (WPA2 preferred) data in transit over the air is protected as far as the router/access point. Once on the router though data in motion is subject to inspection by whoever controls the router, the ISP etc. unless it is encrypted by the handheld (BES, https or tls/ssl (with deviceSide=true)).
As far as someone on the Wi-Fi router, or on the Internet accessing and changing the router, that depends entirely on how well the security on the router is implemented.
My other Blackberry is a PlayBook.