It seems that most prepaid platforms are not able to distinguish between having a domestic bundle active or not in order to apply different roaming price. Therefore most operators seem to set prices for prepaid roaming from May on somewhere at the EU provided surcharge amounts without adding any domestic base price.
I do not think that O2 Germany will be able to charge 0,23 € / MB in EU-roaming as almost all their prepaid tariffs include domestic allowances which means that O2 is not allowed to charge more than a surcharge which in Germany is not allowed to reach 0,06€/MB.
The regulated prepaid roaming price differences between the different EU operators seem to be minimum. The Spanish operators Orange and Movistar managed to surpass the allowed amounts applying the rounding up, strictly forbidden by the EU regulation. Vodafone UK seems to have one of the better offers on its PAYG/prepaid with free incoming calls:
EU(except UK) to EU call: 4p/min
EU(except UK) to EU SMS: 1p
Incoming calls in the whole EU: free
Data in the EU (except UK): 4p/MB
http://www.vodafone.co.uk/campaigns/...-reg/index.htm
On the other hand, the existing alternative prepaid roaming offers (daily/weekly allowances) unfortunately seem to stay unmodified in most of the cases also after the April 30th.