>I'm using some TI CAN transceivers on a network here. The idle state
>is both lines at 2.5 volts (5V bus). For the active state one line
>goes to 1.5 volts and the other 3.5 volts.

This is illustrated in the Philips application note (AN96116) as well. It
seems that this is the normal way for the "fault protected" chips to work,
and there are some more complex chips around (e.g. Maxim MAX3054/5/6 and
Philips TJA1054) which are also termed "Fault Tolerant" and will carry on
working even with one line connected to the vehicle battery or ground. These
have more pins on them and use somewhat more external components which
actually pull the two lines to Vcc and Gnd so there is a reverse voltage
across the inputs of the receiver, instead of floating at Vcc/2. Seems to
depend on how fault tolerant you need as to which chips you use.

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