This isn't a hardware issue - unless you consider equipping a Sense 3.0 phone with less than 1GB of RAM a hardware problem - it's simply that the Sense 3.0 launcher is so big that Android can't keep the whole thing in RAM all the time; occasionally Android has to eject it to make room for the Market app, or some other; and then when you return to the Home screen, the launcher has to be reloaded.
Sending it back to the supplier and getting a replacement won't help, once you've set up the new phone and been using it for a few days.
All recent HTC Sense 3.0 phones have the same problem, because they all have only 768MB of RAM, which isn't quite enough. My Desire S never had a restart problem with Sense 2.1, but it happens a lot with 3.0.
Other 3rd-party launchers:
1. Usually have an option to keep them resident in memory.
2. Are smaller, so are less likely to be swapped out anyway
3. Are smaller, so if swapped out, they take less time to reload.
(I realise I've repeated myself a little in that list, but the launcher size is so important, it's worth repeating).
It is possible to build the HTC launcher with a flag to stay resident, and the people over at xda-developers have already done it for those with rooted phones; I'm surprised HTC hasn't done this itself in an official update yet - actually no I'm not - they probably hope it'll encourage people to upgrade to the newer 1GB devices being announced at MWC this month.
The only way around this is either to root your phone and use a custom Sense build configured so the launcher stays in memory; or to keep your stock ROM and use a different, smaller launcher.
I've dabbled with Launcher Pro, but have settled on GO Launcher Ex, because the free app has more features. It's a little slower than Launcher Pro, but both are faster than Sense, and I now suffer no launcher restarts.
The only downside is that I cannot use the Sense weather/clock widget, but there are free alternatives which are nearly as good.