Its obviously not an operator setting if you are using the
same sim with two different results in two different devices.

It could be either.  But if you're using the same SIM, it's almost
certainly the Centro.

Handsets do need to support concatenated SMS for it to work.

Does the Centro allow you to enter more than 160 characters?

A minor point, but a single unsplit message is limited to 160
chars whereas the parts of a split message are limited to 153
chars each (limited by referencing overheads).

For whatever reason, it's the device that's not playing ball.
The operator doesn't care (but more message parts = more
profit).

The device does the splitting into individual message parts (and
the cross-referencing), and submits each part to the operator
as a complete and separate PDU-encoded payload.  Checking for
well-formedness by the operator will be very rudimentary.  The
receiver reassembles the long message (if it's smart enough to
do so) according to the referencing info in the parts' headers.

There's inconclusive discussion about this Treo issue, eg here:
http://tinyurl.com/5mdl6t

There's some talk there about different SIMs affecting some
Treo's willingness to accept and split long messages.
Technically, this makes no sense to me (unless the SIM's SMS
storage capacity is near-full perhaps).  But you might want to
experiment.  I'd be interested in any findings.

John