Run these commands to import the correct keys. 2007.0 in the url is correct, even if you use Spring 2007.1.

curl ftp://distrib-coffee.ipsl.jussieu.fr/pub/linux/MandrivaLinux/official/2007.0/i586/media/main/release/media_info/pubkey > mainpub
curl ftp://distrib-coffee.ipsl.jussieu.fr/pub/linux/MandrivaLinux/official/2007.0/i586/media/contrib/release/media_info/pubkey > contribpub
rpm --import mainpub
rpm --import contribpub
rm mainpub contribpub

Then edit /etc/urpmi/urpmi.cfg and replace the "key-ids: 26752624" in media Main with:
key-ids: 70771ff3

and of media Contrib with:
key-ids: 78d019f5

This should do it.