First, let's define "kernel". The kernel, in Android terms, is the core Linux OS that the Android framwork and Dalvik Virtual Machine run on top of (those 2 things are what make Android, Android.) In the case of the stock Bionic, the Linux kernel is v2.6.35.7
Now, let's talk about "alternate" kernels. There are 2 things that could be considered an "alternate" kernel.
First is the "custom" kernel, which is not pertinent to the Bionic due to the locked bootloader. These are created by members of the community from source code, which is downloaded from Google's servers. I used to build custom kernels, and worked with P3Droid back in the early days of root and the Droid 1. Basically, I had to install Linux on my PC and set it up for compiling C++ source code. I used VMWare so I didn't have to have a Linux partition. Then, I had to download the Android kernel source. From there, modifications could be made to do such cool things as overclock the CPU, undervolt the CPU, monitor the temperature, tether wirelessly, and the list goes on. Some of this involves direct modification of the C++ source code, other things involve using a configuration editor to toggle options. Once you finish with all of this, you compile it on Linux and copy it to your device. If all goes well, there ya go, custom kernel. But like I said, unfortunately not pertinent to the Bionic unless the bootloader is eventually unlocked.
Second is the kernel update, which is packaged with carrier OS updates. Generally, when the Android component of the OS is updated, they update the Linux kernel as well. That's what are referred to in threads such as "5.7.893 radio/kernel"; 5.7.893 is the "system version", which is the Motorola-issued version for the entire OS package (kernel, radio, and Android OS). I am not sure what the kernel version of .893 is, but it's likely later than 2.6.35.7 (something like 2.6.35.21, for example). "Radio" refers to the firmware revision of the actual cellular hardware and updates to this are also included in OS updates. Since we're discussing the kernel here, a kernel update within an OS update is like the kernel updates you see running desktop Linux distributions such as Ubuntu. Generally, they are sort of like Windows security updates, where critical issues are fixed.
Some people swear by some of the new, non-official updates, claiming improved network performance/stability. But this is a function of the radio firmware, and not the kernel. Since there is no way to update the kernel on your Bionic without installing an OS update that might take you off the official update path I would recommend against it currently.
