Until ROM Manager gets the manifest issue sorted out, here's how you can use it to grab the latest aospCM7 build.
1) Install ROM Manager
2) Open ROM Manager
3) Press your menu key
4) Select Manual Flash Override
5) Press OK at the Recovery warning
6) Scroll down and select the LG Optimus One (not the Optimus V)
7) Select ClockworkMod 3.x
8) Press OK at the popup
9) Press Download ROM
10) Select aosp (even though it says 5/31/2011, just select it)
11) Select aospCM0d-VM670-20110601-MMSFIXED
12) Select Download
13) At the ROM Pre-Installation popup, press Cancel
14) Quit ROM Manager
15) Use a file manager app to go into sdcard/clockworkmod/download/romshare.deployfu.com/downloads/322/255
16) Copy or cut&paste aospCMod-VM670-20110601-signed.zip to the root of your sdcard
17) Quit file manager
18) Reboot into recovery (you should have clockworkmod 3.x.x.x installed already, if not see below)
19) Do a backup (nandroid)
20) Wipe cache and dalvik cache
21) If you are coming from another ROM, format boot, system, data, and cache
22) Install zip from sdcard and select the aospCMod zip file
23) Install zip from sdcard and select gapps (see below if you don't already have it)
24) Reboot system
25) Enjoy
If you don't already have clockworkmod 3.x.x.x installed, download it to your sdcard, open Terminal Emulator from the market, then type in
flash_image recovery /sdcard/NAME_OF_CLOCKWORKMOD.img
Here is the recomended recovery for aospCM7:
aospCWMod-VM670-20110531.img
If you don't have gapps, download it to your sdcard, then install after flashing the ROM
Here is the gapps:
gapps-gb-20110325-signed.zip