Recent changes in Debian sid indicate that /tmp is now mounted as tmpfs by default, being 20% of RAM available.
# man 5 rcS RAMTMP Make /tmp/ available as a ram file system (tmpfs). Set to 'yes' to enable, to 'no' to disable. It defaults to yes for new installations (wheezy and later); no for upgrades from earlier releases, which do not contain RAMTMP in /etc/default/rcS. The size of the tmpfs can be controlled using TMPFS_SIZE and TMP_SIZE in /etc/default/tmpfs. However, /etc/default/tmpfs is deprecated and to override the defaults an entry should be created in /etc/fstab, for example: tmpfs /tmp tmpfs nodev,nosuid,size=20%,mode=1777 0 0 Packages can not expect directories in /tmp to exist after boot.
Various applications put more into it, so it may result in problems.
The actual setting happens within /etc/default/rcS
is the key to be changed, if you do not want that behaviour. If the size needs to be adjusted, add a fstab entry instead.
Update 2012-12-30: This has been disabled by default. See follow-up here.