When someone buys a country’s government bond, the government needs to pay it back on a specific date. If the government refuses to do so, there will be a total collapse in international confidence in the debt of the country and of all its banks, companies and residents, a currency crisis, and generally a wide range of Very Bad Consequences.
When someone works for a country’s government and is told they’ll get a nice fat pension, the government can decide to not pay it. This will upset the person who works for the government quite a lot, and might be perceived as somewhat unfair. However, the consequences for the country’s ability to borrow, invest and trade in international markets are non-existent.
Therefore, anyone who counts public sector pensions as a liability faced by the UK government in the context of the current financial crisis (generally as a way of saying ‘ooh, we’re nearly as bad as Iceland’ – Nadeem Walayat, this means you) either doesn’t understand what they’re talking about, or is trying to mislead their audience into thinking things are much worse than they really are.