If you're in a pinch I guess you could use the image contained in the Recovery partition to install.
To mount the recovery partition do :
Mac-mini:~ tyr$ diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *320.1 GB disk0
1: EFI 209.7 MB disk0s1
2: Apple_HFS MiniHD 305.4 GB disk0s2
3: Apple_Boot Recovery HD 650.0 MB disk0s3
4: Microsoft Basic Data WINDOWS 11.4 GB disk0s4
Mac-mini:~ tyr$ diskutil mount /dev/disk0s3
Volume Recovery HD on /dev/disk0s3 mounted
The image is called "BaseSystem.dmg", and the image itself contains a Lion installer :
Mac-mini:~ tyr$ ls -l "/Volumes/Recovery HD/com.apple.recovery.boot"
total 930040
-rw-r--r--@ 1 root admin 1876 Jun 30 08:55 BaseSystem.chunklist
-rw-r--r--@ 1 root admin 451307798 Jun 30 08:47 BaseSystem.dmg
-rw-r--r-- 1 root wheel 2245 Jun 16 03:06 PlatformSupport.plist
-r--r--r-- 1 root wheel 475 Jun 30 05:42 SystemVersion.plist
-rw-r--r-- 1 root wheel 858800 Jun 30 08:04 boot.efi
-rw-r--r-- 1 root wheel 361 Jul 21 10:14 com.apple.Boot.plist
-rw-r--r-- 1 root wheel 23992189 Jun 30 07:41 kernelcache
Mac-mini:~ tyr$ hdiutil attach "/Volumes/Recovery HD/com.apple.recovery.boot/BaseSystem.dmg"
Checksumming Driver Descriptor Map (DDM : 0)…
Driver Descriptor Map (DDM : 0): verified CRC32 $A505B8BE
Checksumming (Apple_Free : 1)…
(Apple_Free : 1): verified CRC32 $00000000
Checksumming Apple (Apple_partition_map : 2)…
Apple (Apple_partition_map : 2): verified CRC32 $B0720C43
Checksumming Macintosh (Apple_Driver_ATAPI : 3)…
Macintosh (Apple_Driver_ATAPI : 3): verified CRC32 $F1E8BA9E
Checksumming (Apple_Free : 4)…
(Apple_Free : 4): verified CRC32 $00000000
Checksumming disk image (Apple_HFS : 5)…
................................................................................ .................................................
disk image (Apple_HFS : 5): verified CRC32 $973623CF
Checksumming (Apple_Free : 6)…
(Apple_Free : 6): verified CRC32 $00000000
verified CRC32 $35F3BC78
/dev/disk3 Apple_partition_scheme
/dev/disk3s1 Apple_partition_map
/dev/disk3s2 Apple_Driver_ATAPI
/dev/disk3s3 Apple_HFS /Volumes/Mac OS X Base System
Mac-mini:~ tyr$ ls -l "/volumes/Mac OS X Base System"
total 24
drwxrwxr-x+ 5 tyr staff 170 Jun 30 07:38 Applications
drwxr-xr-x 3 tyr staff 102 Jun 30 04:24 Install Mac OS X Lion.app
drwxr-xr-x+ 15 tyr staff 510 Jun 30 07:38 Library
drwxr-xr-x+ 4 tyr staff 136 Jun 30 07:21 System
drwxrwxrwt 2 tyr staff 68 Jun 21 08:31 Volumes
drwxr-xr-x 39 tyr staff 1326 Jun 30 07:41 bin
drwxr-xr-x 2 tyr staff 68 Jun 18 21:44 dev
lrwxr-xr-x 1 tyr staff 11 Jun 30 07:38 etc -> private/etc
drwxr-xr-x 5 tyr staff 170 Jun 30 07:38 private
drwxr-xr-x 61 tyr staff 2074 Jun 30 07:41 sbin
lrwxr-xr-x 1 tyr staff 11 Jun 30 07:38 tmp -> private/tmp
drwxr-xr-x 8 tyr staff 272 Jun 30 07:24 usr
lrwxr-xr-x 1 tyr staff 11 Jun 30 07:38 var -> private/var