In April, AT&T ended a policy of offering data feature packs to its lowest end Pay as You Go plans (specifically, the $1/day and the $0.10/minute plans). At that time, TUAW was told by an AT&T spokesperson that customers on those plans could still pay a pay-per-unit (PPU) rate for data, approximately a penny for 5Kb, if I have my numbers right.
Although most users would not welcome paying $1536 for a 750MB movie, PPU data would allow them to at least check their email on the go, albeit at a more expensive rate than they'd been used to.
Yesterday, TUAW started hearing that users were unable to access PPU data on their handsets. We are unsure if this is because users need to update their configuration profiles or if the PPU system itself is experiencing difficulties.
I personally tested on my 3GS handset with both 3G and Edge access (10c/minute GP01 plan) and was unable to access the Internet at all.
TUAW has contacted AT&T and asked for them to look into the matter. If and when they reply, I will update this post.
Are you able to access PPU data on your PayGo phone without opting into monthly plans? Let us know in the comments.