PubMed has announced that MEDLINE records would in many cases now refer to the online, electronic versions of journals rather than the print version. This reflects the fact that many publishers now deem the ejournal as the version of record with complete content for each issue. More about PubMed practice here
In parallel with this development, publishers such as BMJ have dropped traditional date - volume - part - page pagination in references and replaced it with a new method where the electronic article is given a unique number instead of page numbers.
BMJ references in in PubMed now look like this:
Should antihistamines be used to treat anaphylaxis?
Andreae DA, Andreae MH.
BMJ. 2009 Jul 10;339:b2489. doi: 10.1136/bmj.b2489.
The date (Jul 10) and volume (339) format remains the same but the article reference is now to " b2489 " a number unique to this article in this issue. Also referenced is the doi or digital object identifier, a unique string which enables a digital object to be searched and found on the internet.