Well, it's not wise to change both the hardware and the software at the same time. You think it will reduce your time to market but it might increase it instead due to the numerous changes that will have to happen in your toolchain before getting anything barely working again.
From what I understand, you have a big experience in 2.4 and Xscale. 2.4 Also works on x86, so you'll not have to re-learn everything from scratch by just changing the architecture. All your toolchains, boot scripts, packaging scripts, etc... will still work as they did before. Then, only once you get familiar with your new architecture and the minor changes you might observe in the boot sequence, build process etc... it will be the right time to evaluate a migration to 2.6. Once you put your finger there, you'll quickly discover that you need to upgrade your gcc, glibc, replace modutils with module-init-tools, experiment with hotplug and sysfs, maybe switch to udev, etc... Step by step you'll notice a big number of changes, but you will be able to proceed one at a time, which is not possible if you change the soft at the same time as the hardware.
Also there are other aspects to consider. 2.4 has been maintained for a very long time, and you're probably used to backport some mainline fixes to your own kernel from time to time. 2.6 is not maintained that long (avg 6 months), and changes so fast that you will not be able to backport fixes for many years. I'd strongly recommend to start with 2.6.27, because Adrian Bunk will maintain it for a long time, as he did with 2.6.16. Once 2.6.27 is not maintained anymore (in about 2 years) you'll have to decide whether you stick to 2.6.27 and try to backport fixes yourself or switch to 2.6.36 (just a guess).
Also, 2.4 accepts almost no new hardware nowadays. If your new platform works well, that's fine, but how can you be sure that next year your GigE NIC will not change to something not supported anymore ?
I would say that the only case where 2.4 would make sense for a long term starting from now is if you don't have the time to revalidate 2.6 or to wait for 2.6.27 to stabilize, and need to quickly release something which will sit at your customer's in a place where it cannot be upgraded. Something like "install and forget". But I don't feel like it's what you're looking for.
So, to summarize :
1) switch your architecture
2) switch your kernel
Whether an official release of your product exists between 1 and 2 is just a matter of your time constraints and customer demand.
Last, to show you you're not alone, I'm too considering switching our products to 2.6, but next release will still be 2.4. Too many changes for a short-term release, and 2.6.27 not ready yet to reach years of uptime (but it's getting better though). 2.6.25 was particularly good but not maintained anymore.
Hoping this helps,