So in trying to create an MBR partition of my hard drive which I could then use to boot a solaris-derivative OS using rEFIt, I messed up. Rather than a hard drive with a primary GUID partition map scheme to boot OS X 10.7 from, and an auxiliary partition with an MBR scheme for non-Mac OS's, I now have a hard drive that strangely enough, is formatted with an MBR scheme. Additionally, the very helpful recovery HD is gone, and I don't think i'll be able to reinstall it (http://apple.stackexchange.com/questions/19145/how-can-i-create-or-recreate-a-lion-recovery-partition), without a normally formatted HD.
In Terminal:
diskutil list
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *500.1 GB disk0
1: Apple_HFS Macintosh HD 500.1 GB disk0s1
In Disk Utility:
Disk Description : APPLE HDD HTS547550A9E384
Media Total Capacity : 500.11 GB (500,107,862,016 Bytes)
Connection Bus : SATA
Write Status : Read/Write
Connection Type : Internal
S.M.A.R.T. Status : Verified
Partition Map Scheme : Master Boot Record
Not surprisingly, this initially caused some problems in that neither Macintosh HD nor the Recovery HD were being seen by EFI, but I was able to erase the hard drive, and restored my OS via Time Machine.
This leaves me in a tenuous situation. Messing with partitions and such is always risky business, and if anyone can come up with a reliable and trusted method for returning my HD to its native GUID'ness, it would be much appreciated! As I have a Time Machine, I'd be totally ok with completely erasing my HD and reinstalling lion.
EDIT:
sudo fdisk /dev/disk0
Disk: /dev/disk0 geometry: 60801/255/63 [976773168 sectors]
Signature: 0xAA55
Starting Ending
#: id cyl hd sec - cyl hd sec [ start - size]
------------------------------------------------------------------------
1: AF 1023 254 63 - 1023 254 63 [ 1 - 976773167] HFS+
2: 00 0 0 0 - 0 0 0 [ 0 - 0] unused
3: 00 0 0 0 - 0 0 0 [ 0 - 0] unused
4: 00 0 0 0 - 0 0 0 [ 0 - 0] unused
sudo fdisk /dev/disk0
(or whatever number your disk is) to the question - depending on the partition layout, there may not be enough space for a GPT. It certainly looks like there won't be enough space for an EFI System Partition, which means you won't be able to do firmware updates on this Mac unless you start over.