I think I've clarified this in the CBA FAQ thread with a link to a past discussion, but I'll (God willing) put this to bed once and for all.
First off, players who are injured continue to count against the cap. The CBA mentions this in at least three places iIRC. Just because a player is injured doesn't mean he quits counting against the cap - even if he goes on LTIR. People keep saying, "Oh, ___ is on IR, so he doesn't count against the cap." WRONG, WRONG, WRONG! Article 50.10 even mentions this ... so when a player goes on IR (or even LTIR), the team doesn't gain any cap space. Period.
What they do get is the ability to add players up to the amount of the injured player while the injured player is on LTIR. So let's pick on Buffalo with a very simplistic (yet real) example: say they're at $43.95 million with a $44 million cap and Tim Connolly (with a cap number of $2.9 million) goes down injured in training camp and may miss a huge chunk of the season. The Sabres can place Connolly on LTIR, but they're still at $43.95 million wrt the cap. The move allows them to be able to call up players whose cap numbers add up to $2.9 million - the first $50,000 is what would get the Sabres to $44 million, and everything else is not counted when calculating the cap because Connolly's LTIR designation absorbs it. It's not just a flat, "Oh - Connolly's out? Woo-hoo, we have $2.9 million free!" situation.
To add to this: a player cannot be placed on LTIR unless the team thinks he'll miss at least 10 games and 24 days and the team has to be in a situation where adding a player would put them over. If the team is at $38 million and a guy making $3 million goes down, tough - they've got space to add guys, the injured player cannot be placed on LTIR. But let's suppose they're near the cap and can put the player on LTIR; then they're charged with whatever gets them to the Upper Limit before the LTIR exemption kicks in.
Now ... what if the Sabres wait until 45 days into a 180-day season before taking advantage of Connolly's LTIR status - can they take use of space they didn't eat up in the first 45 days? No - it's lost. Unlike the cap in normal situations where you can bank saved payroll space for later, there's nothing being saved here; remember, even without calling anyone up the Sabres are at $43.95 million with Connolly on LTIR. They can't save the $2.85 million they could have used for the trade deadline - if you don't take advantage of potential LTIR exemption space when it's available, you lose it.
And as kdb points out, to bring the injured player off LTIR they must clear enough cap space to have him on the active roster and not be over the Upper Limit.