The high definition (HD) television channels on satellite require a lot more bandwidth (number of bits per second) than the standard definition (SD) ones, as you might expect.
However the additional capacity required is reduced in two ways. First the MPEG-4 compression system is used (rather than MPEG-2) and generally the DVB-S2 modulation system increases the bandwidth of each satellite transponder.
Generally speaking, around nine SD television channels can be carried on a transponder in DVB-S mode, and four HD channels on one in DVB-S2 mode.
For historical reasons, the BBC's Astra 2D transponder 50 has been providing BBC One HD and BBC HD (and also ITV1 HD Merdian South) in the DVB-S mode, and this has used all the capacity on the transponder, because of the lower bitrate.
One 6th June 2011, the BBC will switch the transponder to DVB-S2. If you watch using Sky+HD, Virgin Media or Freeview HD you will not have to do anything.
If you use a branded Freesat box (or TV set), you may need to put it into stand-by mode for 30 seconds on 6th June. If that doesn't work, perform a 'first time installation'.
Update: Some Panasonic Freesat boxes are unable to deal with the change - see comments below
If you are using a unbranded Freesat box, you may find that you also have to do a 'first time installation' to get the channels back.
Some older satellite equipment will not support DVB-S2. If you can't watch Channel 4 HD, then you will no longer be able to watch the BBC HD channels.
Sky HD, Freesat HD EPG issues
Currently BBC One HD cannot automatically replace BBC One in the Sky EPG at position 101, because there are 15 regional versions of BBC One, and only a single BBC One HD 'stream' on the satellite.
BBC Two carries four regions for England, Scotland, Wales and Northern Ireland. For a BBC TWO HD to appear in the Sky EPG at position 102, the BBC would need to provide four versions of BBC TWO HD.
The extra capacity found by moving to DVB-S2 could allow the BBC to provide BBC Three, BBC Four, CBeebies and CBBC channel in HD, and these could be provided in the "usual" EPG slots for Sky HD and Freesat HD users, which would put them on the lower numbered channels nearer the top of the guide.
SteveD: Have a look in your TV's user manual / specifications and check under tuner if DVB-T2 is seen listed, because if its only DVB-T then its not capable of picking up an HD signal, or alternatively this aspect can be checked out if you provide the model number of the set.
However if you have seen the TV listed as "HD Ready" then this only means that its capable of producing HD images and "not" that it can actually receive them via an aerial, the HD images being from an external device, e.g: HD set top box coupled into the TV via the HDMI input.
The "HD Ready" definition being something I disagreed with right from the start as it was just bound to cause confusion amongst potential purchasers of these devices, and indeed already has done to many!
SteveD: Well, the TV is capable of receiving HD transmissions but only via Freesat, and as you mention reception via Freeview is only in standard definition. Of course there are one or two other brands (Samsung or LG) with similar odd ball combinations like that.