What ways are there to move my addressbook to the iCloud?

You need Lion/10.7.x on the Mac to be able to sync AB, or sync anything.

I have no idea what an iPad can do, but at 10.5.8 on the Mac, the only possible thing I can think of is to setup an iMap email account on the Mac & iPad, then in AB select all & export one big vCard, then email that vCard to the iPad.

iCloud Mail setup...

IMAP (Incoming Mail Server) information:

• Server name: imap.mail.me.com
• SSL Required: Yes
• Port: 993
• Username: example@me.com (use your @me.com address from your iCloud account)
• Password: Your iCloud password

SMTP (outgoing mail server) information:

• Server name: smtp.mail.me.com
• SSL Required: Yes
• Port: 587
• SMTP Authentication Required: Yes
• Username: example@me.com (use your @me.com address from your iCloud account)
• Password: Your iCloud password

I have no idea how to unpack a huge VCard

Not sure on the iPad, but on the Mac it'd be Address Book>File>Import>vCards