That's 50% true. However, the handset and the computer can't be in the same area either. That means that yacking on the wireless phone at the same time as you're pounding on your wireless laptop is not going to work.
Different models have different problems. If it's an FM 2.4GHz (non-spread spectrum) type of phone, it will probably work just fine in the same room as the WRT54G. If it's a DSSS (direct sequence spread spectrum) cordless phone, then it will slow down the wireless, but not kill it. If it's a FHSS (frequency hopping spread spectrum) cordless phone, it will kill the wireless. Since you didn't bother specifying the maker and model number of your phone, I can't offer any additional guesswork.
Well, up until about a month ago, I would have said that they only interfere when being used. My Panasonic GigaRange KX-TG2257 2.4GHz phone is probably DSSS and only causes problems when I'm yacking on the phone. However, there has been one report in this newsgroup that there are some models where the phone base constantly transmits, even when there is no phone conversation. I'm not sure I believe this, but it is possible.
Standard answer: Try it and see if it works. If there's a problem, it will be obvious.