Can you put a laptop into a bag or case while on standby?

If it's only going to be a few minutes then no problem. If it's going to be 30 minutes or so then _I_ wouldn't. It still creates a bit of heat and when you have it in a bag/case you are actually insulating it and it will store all that heat.

Another reason is some times in standby or sleep mode some computers or HD's won't have the HD "arm" locked away into a neutral position where it won't damage the HD platters them selves when bounced around. You should never move a Laptop around while it's on, asleep or on standby for this reason. Moving it around on your lap is one thing but traveling in a bag or case is another thing.