So, in short words.....

It may be possible, not advisable, do it at your own risk, and the best way is to engage the clutch, off the throttle, shift gear, and resume acceleration.. A.K.A. the good old way... :driving:

There is an aftermarket piggy-back kit that can allow for this (drive by wire only). You can program the delay for your normal shift time. When the clutch is engaged, the piggy-back reduces the throttle and then goes back to WOT after the programmed delayed. It is more beneficial for a turbo car because you can have the system keep the boost on....or at least help keep the turbo spooled. For an NA car, it just seems like being lazy.

EDIT----The piggy-back pulls ignition, not throttle. Oppss.... Here is the link....
http://www.npcompleteperformance.com/wotbox

Interesting. I may look into getting something like that for my Solstice. I tried my hardest to no-lift with the Solstice because that turbo winds down fast. This could help. Although, for $300, GM has an add-on kit that removes that limitation from the computer and allows the waste gate to hold more boost (stock will only allow 22PSI). Not really worth the money, but it's warrantied.

Used to this on my bike on the track, though it's quite necessary on the street.

Its my understanding that this isn't great to do with cars. Something to do with synchro mesh or what have you. I know it's the preferred method for large commercial trucks but they have a different set up compared to cars. One has synchro mesh and one doesn't, can't remember which is what, etc.

On the flip side I do this on my bike from time to time but it's much safer for a sport bike. All our gears are actively engaged just not all of them under load. It's as simple as just clicking in to gear. Lots of guys use this method at the race track because it saves time off their laps if you are good at it.