If you have a 3G iPad with a Vodafone prepaid plan you'll most likely be going to their "/ipad" page to review your data usage and to top-up your account. This has been working fine for me. Last week I updated my iPad 2 to iOS 5. Yesterday I went onto the Vodafone page to top-up my account:

I enter my credit card number and move to the next field. When I do this my number is formatted with commas and rounded! For example, enter 1234123412341234, move to the next field and the number changes to 1,234,123,412,341,230. Use "1236" as the last group and it rounds up to "1,240". The year is also formatted with a comma. End result, I cannot top-up my account through the usual page. Fortunately Vodafone added my iPad to my cellphone account, so I was able to add funds using my PC. I have tried playing with the internationalisation settings on my iPad, but it didn't resolve things.

I've reported this to Vodafone and they've spoken to Apple tech support. Apple wants to talk to me. Just thought I'd post this here in case other iPad owners are running into the same problem.