- text messages are capped at 0.11 EU
- data is capped at 1 EU / Mbyte (0.50 EU in 2011)
- calls are capped at 0.46 EU for outgoing and 0.19 EU for incoming (0.35 EU and 0.11 EU in 2011)
- per-second billing is mandatory after the first 30 seconds
- providers are required to let customers set a cut-off amount (50 EU by default) for their charges
These limits apply regardless of contract, including prepaid pay-as-you-go plans.
Note that these are better conditions than some domestic US plans; for example, AT&T's PayGo (no commitment, pay-as-you-go) is $0.25/minute (in or out), $0.20/SMS, and $10/Mbyte. In different words, it's cheaper to make roaming international calls within Europe than to make domestic calls in the US. And no major US carrier that I know offers cut-off amounts on anything other than pre-paid plans. Maybe the US needs more regulation of its mobile providers?