The DULG DebuggingUBoot page has this to say (the "relocation" it speaks of is copying itself from flash to RAM).
For debugging U-Boot after relocation we need to know the address to which U-Boot relocates itself to. When no exotic features like PRAM are used, this address usually is - CONFIG_SYS_MONITOR_LEN. In our example with 16MB RAM and CONFIG_SYS_MONITOR_LEN = 192KB this yields the address 0x1000000 - 0x30000 = 0xFD0000.
Further reading of the text seems to indicate it's dependent on the processor or board and that you may have to check the U-Boot source to know for sure.
Regarding the Guruplug:
On the RAM side, u-Boot has the first 8 megabytes reserved. The rest is free. Some developer’s will load kernels and filesystems at 0×800000 for programming to flash. Another common spot is at the 100 megabyte boundary (or offset 0×640000).
Depending on your version of U-Boot commands may be available to put a string somewhere in free RAM and then hunt the rest of RAM for that string, revealing the approximate location of U-Boot in memory.