Hi,
I haven't tested this and therefore can't be sure, but I think that as long as you select the indented Macintosh HD "volume icon" in Disk Utility as the item to be erased, then the hidden Recovery HD partition should be preserved:
Do not select the non-indented "drive icon" above it - if you do then all the partitions will be erased, including the hidden Recovery HD partition.
By way of example, in my own system I have Lion installed on a partition on an external drive. If I enable DIsk Utility to show hidden partitions, that drive looks like this:
If then I select the indented Lion volume "Lac5 BetaLion" and click "Erase," I get this warning dialog:
I didn't proceed with this, but that dialog suggests that the similarly indented "Recovery HD" partition would not be be affected. By contrast, if I select the upper non-indented "drive" icon and click "Erase", I instead get this dialog:
Perhaps others here have actually tried erasing just an indented Lion partition, and can comment further as to whether the Recovery HD partition is preserved.
edit:
I was actually booted in Snow Leopard when I tried the above in Disk Utility - I haven't looked while booted in Lion.
Message was edited by: jsd2