My Samsung TV LE40D580 has built in Freeview HD, but we nearly always watch via a Sky+ HD box.
Sometimes when switching on, the TV source has switched to Freeview, even though previous viewing was via Sky.
Samsung say this is normal, but as it does not always occur, this seems strange.
I know it is easy to change source, but irritating.
Is there any way to lock on to the Sky source?

On our Sony HD TV we have both Scart & HDMI inputs - the source is automatically selected from the Scart inputs, so when we switch the TV on we get terrestrial TV but when we switch on the Sky (or freesat) box it automatically switches to that input, since both boxes are connected via scart leads (the freesat HD box is ALSO connected via hdmi but one has to select the relevant AV input on the TV remote to view any HD programme in full HD 1080 format).
I wonder if your Samsung is the same....

I thought if you connect the Sk box to TV by HDMI and SCART that the SCART would take priority and you would be watching in SD. Are you saying that your TV allows you to select HDMI or SCART?

Don't all TV's do this - if they have the sockets [AV1/AV2/HDMI1/HDMI2 ...]??

Most TVs do not auto-switch to an active HDMI source, it is one of the bugbears often commented upon by people use to this with their Scart devices.

My Samsung TV always turns on switched to the same source as was last used before it was turned off.

Yes it does, so one has to select the relevant hdmi input to view the TV source in HD.
(our 4 year old Sony has two hdmi inputs and three scarts)

Yes mine does also 95% of the time, but there seems to be no reason when it switches to Freeview.