I was unable to get "wireless Dial-up" to work. I called up Cingular and told them that when I signed up for Media Works Unlimited GRPS data, the CSD data was removed from my plan. I explained that I understood that this is normal but asked them if they could add CSD back to my account. The customer service rep gladly did so and said that "it's normally $3.99/month, but I'll put it on for free since you're on media works."

With that she added CSD circuit swiched data back to my account.
Once I had that on my account I can now use "wireless dial-up" connection on my treo.

Here's what that looks like on my rate plan:
Feature Package HOME Wireless Internet - Circuit Switch Data $0.00

You must re-set your Treo after you have connected with GPRS before dial-up will work. I don't know why, it must be a bug. I discovered this by accident.
But if you EVER make a connection (and disconnect) via GPRS, you must re-set the treo, then your wireles dial-up connection will go through. If you don't, it just won't work.

This may be why so many people have had trouble making it work, they don't know they need to re-set the phone first.
For that matter, i don't know if adding CSD back to my accound had anything to do with it, it may have been that I just never tried re-setting my treo first.

By the way, that "CSD" feature also allows the "ISDN" connection which is faster and also uses your minutes just like dial-up. It's network connection called "Cingular" (CSD/ISDN) and "Cingular Express" is GPRS.

FYI, on cingular, my samsung phone had 3 network connection profiles pre-programmed:
Cingular (CSD, aka the old way of doing data. ISDN)
Cingualar Express (GPRS)
Roaming (Wireless dial-up)

The cingular (ISDN) and roaming (dial up) both connect to a 1415xxxxxxx number (I don't remember it off the top of my head.) so you don't have to use your own ISP if you don't want to.