GSM cell phone with RS232 data interface ??

Most of the USB data cables I have examined are USB-RS232 converters. I daresay the majority of cellphones still have "RS232" (meaning async

3-wire serial, with 3.3V or 1.8V inverted signal levels.

Why not get a GSM module which has serial port normally TTL like levels see for example

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I was searching for one of these as well at one point, but all the phones I found had been discontinued. You could always use one of these:

They're a bit expensive but they seem to work well (except for a quiet but irritating hissing noise coming from the power supply).

If you're planning on building many devices, then you might consider using a GSM module. They're cheaper but you have to add your own antenna, power supply, SIM socket, enclosure, etc.

Some of the low-end motorola phones sold recently as AT&T go phones had that feature, i think it was through the headphone jack.

Yes. You want something like a Wavecom m1206b modem.

Just to follow up cheapest place I found for RS232 GSM modem is:

You can get one for £25.