I phoned with Arno Pirner @Hetzner since there was no reply to my question via mail. The answer was simple and clear: custom IP-Routing (shared IP) will be available in the next Datacenter which is planned to be ready April 2009. Most possibly it will then be available only with an additional (paid) option.
I'm in contact right now with ovh.de, the upper-class servers are definitly able to share IPs (they have 16 or more ips each which call all be shared). Even the smallest "istgenug" (eng: is enough) servers for 25 Euros/month have a failover-IP assigned. It can be shared, managed by their web interface.
Until now, they did not give a definitive answer if it can be shared via script (heartbeat etc.), Ill test it as soon as I can get two servers from there.
Their cheap servers sometimes have issues, which would be no problem in a small failover-cluster of 2 isgenug (available with raid 1 also for 36 Euros). The bigger systems seem to be have better overall stability for a still decent price.