A quote from EUobserver, 5 November 2007:

Legal experts now fear that if Belgium fails to have a new government by 13 December, the outgoing government, normally only charged with 'current affairs', will not have the powers to sign the [new EU treaty].

"The concept of ‘current affairs’ concerns a category of non-written legal rules, constitutional habits; and it is accepted that in general it covers three situations," legal experts Carine Doutrelepont and Pascal Lefèvre wrote in Belgian daily Le Soir.

These three situations are the day-by-day decisions of average or little importance, more important decisions that are the result of commitments made earlier, and urgent matters which need to be regulated immediately to avoid serious damage to the common good.

According to the experts, putting a signature to the Lisbon treaty is unlikely to fall in any of these three categories, although it could be argued that a signature only implies finishing a process begun much earlier.

[…] It was agreed by member states that the [new EU] treaty would be ratified at the latest by 1 January 2009, but any delay of the signing moment could mean that the treaty would come into force later than planned.

 
Postcript (by Paul Belien)

Apart from the question whether the Belgian Prime Minister is allowed to sign the treaty in Lisbon next December, there is also the matter of ratification. According to the Belgian Constitution the new treaty will have to be ratified by the Belgian Federal Parliament (House and Senate), but also by the various regional parliaments, i.e. the Flemish, Walloon and Brussels parliaments. This process is likely to take more than a year.

The European Constitution, which was signed in Rome in October 2004, was ratified by the Belgian Senate in April 2005 but it took until February 2006 before the last of the regional parliaments (the Flemish one) had ratified it. The Flemish secessionist and Euro-sceptic Vlaams Belang party (VB), which is the biggest group in the Flemish Parliament and chairs the Foreign Affairs committee, delayed the vote as long as possible.