A lot of it depends on your exact job and the deal between HP and DHL. If you job involves systems that are unique to DHL, then you should be safe for a while. If it is a job like looking after a Windows server or Oracle DBA, I might start looking.
If HP is going to start taking possession of DHL's servers and systems and moving them to their own server farms, then I'd start looking right away. If not, you'll find that much of your time in the future is spent training others how to do your job. That's what happened in my last job.
Keep in mind that HP is buying EDS and there is going to be some rationalizing there. Even without the merger, these companies are looking for the cheapest IT labour possible, which means they are going overseas. So I would not count on any future in HP.
Also oursourcers love to outsource. You job might not even be done by HP: it might be somebody they've contracted to do it. I once had a problem that involved five separate companies, five levels of outsourcing. It felt like I was working for the government or something.
If you hang in through the transition, be prepared for the effect this will have on your relationship with your non-IT colleagues at DHL. I found this really stressful when it happened to me. I could not talk to anyone in our company unless they went through the outsourcer first. Then the outsourcer might not assign the task to me. I used to be the only computer guy for one of our sites. When somebody needed a new program installed on his computer, he called the help desk. They assigned it to a company they outsourced desktop maintenance too. I then had to show the person who came in from 100km away how to install the software on this person's computer. After a while this changes your colleagues relationship with you: you become more and more of an outsider. Soon they start to check to see if you might have a better office than they do.