Update: If you head to the source link at the bottom of this post you'll see updated kernels for other X+ variants available as well. (Taiwan, Hong Kong, Europe, etc.)
HTC has released the kernel source for the AT&T version of the One X+ (that is, the US LTE model). You can find the kernel source (version 1.15.502.9) directly here or at the link below.
HTC Dev Center
[Update: More X+ Kernels] HTC Publishes AT&T One X+ Kernel Source was written by the awesome team at Android Police.
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