Personally my phone doesnt go above 34 when charging - but i always charge via usb on pc (so i can leave it on overnight and it doesnt charge up too quickly)
iv had it go around 42 while gaming for a while on it, but its not to bad.
Just to add , if your worried about it.
HTC has a built in failsafe which will restart if it getting too hot, to stop
any damage, so dont worry about it.
SG2 apprently also suffers from a similar issue.
Why geeks like computers: unzip, strip, touch, finger, grep, mount, fsck, more, yes,fsck,fsck,fsck,umount, sleep.
HTC One Silver 32gb 
Nook HD+ 9 [CM10]
HTC Sensation [Vodafone, running cm10]
HTC Desire [T-mobile , Rooted, RCMixHD Rom]
HTC Diamond [Using Android 1.6 and windows 6.1]